喀麥隆喬立雷-班約剪切帶之板塊演化史

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(1)國立臺灣師範大學地球科學研究所碩士論文. 喀麥隆喬立雷-班約剪切帶之板塊演化史 Tectonic evolution of Tcholliré-Banyo Shear Zone (TBSZ), Cameroon. 指導教授：李通藝. 博士. 共同指導：葉孟宛. 博士. 研究生：楊雅淇. 中華民國一百零三年一月.

(2) 摘要位於喀麥隆境內的班約-喬立雷剪切帶 (Tcholliré-Banyo Shear Zone, TBSZ)是一個東北-西南走向的剪切帶，在喀麥隆屬於非洲中部剪切帶 (Central African Shear Zone, CASZ) 的分支。位處非洲三大地塊：西非古陸塊 (West African Craton)、剛果古陸塊 (Congo Craton)、與東撒拉古陸塊 (Eastern Sahara Block) 之交界處，其構造演化史可以完整記錄此區域相關板塊聚合與分離等板塊演化史。根據本研究於喀麥隆所採集之兩個片麻岩 (10CC02C、10CC05)、兩個糜稜岩 (10CC03A、10CC03C) 與花崗岩 (10CC04) 之鋯石鈾-鉛定年結果顯示：主要年代集中於 1.8 ~ 2.1 Ga 與 696 ~ 514 Ma 之間。而其鈾釷比值數據顯示：片麻岩 (10CC02C、10CC05)、糜稜岩 (10CC03A、 10CC03C) 之核心鋯石 (1.85 ~ 2.14 Ga) 為火成鋯石 (Th/ U＝0.12 ~ 1.23 > 0.1)，但其外圍環帶 (696 ~ 514 Ma) 則為變質鋯石 (Th/ U＝ 0.0013 ~ 0.0098 < 0.01)，且其協和線之下交點約位於六億年前處。雖然花崗岩 (10CC04) 之鋯石鈾-鉛定年結果亦顯示相同之年代區間，但只有非常少的鋯石顯示較老的核心年代，大多數的鋯石顯示核心與周圍環帶記錄了相同之年代區間 (658 ~ 501 Ma)。除此之外，其鈾釷比值皆大於 0.1 (Th/ U = 0.12 ~ 1.17) 顯示此花崗岩成岩於 618 Ma ， I.

(3) 而其較老之核心為繼承鋯石。為釐清不同火成事件之板塊架構與其岩漿來源，本研究亦進行了全岩主要及微量元素分析與鋯石鉿同位素分析。除花崗岩之外，其他岩石樣本之微量元素皆遠低於正常火成岩之含量，這指出班約-喬立雷剪切帶內之片麻岩 (10CC02C、10CC05)、糜稜岩 (10CC03A、 10CC03C) 之原岩為沈積岩。其沈積物質可能源自於伴隨古元古宙時期之埃伯尼造山運動 (Eburnean orogeny) 所形成之火成岩。雖其確切沈積事件之時間還無法確定，但可能與中元古宙時期形成的雅溫得統 (Yaoundé series) 及隆盆地 (Lom basins) 之年代相當。花崗岩樣本之鋯石鉿同位素 (εHf (T) values: －18.5 ~－11.2; +1.0 ~ +10.5) 顯示其岩漿包含地殼與地幔來源之混合訊號，因此可推測此伴隨東撒哈拉古陸塊 (Eastern Sahara Block)、西非古陸塊 (West African Craton) 與剛果古陸塊 (Congo Craton) 碰撞之泛非洲造山運動所形成之火成事件處於一弧陸碰撞之環境，且同時伴隨著角閃岩相之區域/接觸變質作用。根據岩石學之分析顯示此區域之變質沈積岩在泛非洲造山運動之後主要經歷了從角閃岩相變成綠片岩相之褪變質作用。鉀長石 (10CC02C) 之氬-氬定年結果顯示此區域之鉀長石由 589 ~ 519 Ma 持續受到班約-喬立雷剪切帶變形作用之影響。 II.

(4) 關鍵字：鋯石鈾-鉛定年、喬立雷-班約剪切帶、板塊演化、泛非洲. III.

(5) Abstract Tcholliré-Banyo Shear Zone (TBSZ), north Cameroon, is a NE-SW trending branch of the Central African Shear Zone (CASZ). The TBSZ situated within the boundaries of three major cratons: the Western African Craton, the Congo Craton, and the Eastern Sahara Block. By conducting geochronology, geochemistry and petrology analyses on ten samples coming from TBSZ and CASZ, this study attempt to decipher the tectonic evolution of this region. The U-Pb zircon ages of gneiss (10CC02C、 10CC05), mylonite (10CC03A、10CC03C), and granite (10CC04) revealed two major age groups of (1.85 ~ 2.14 Ga) and (696 ~ 514 Ma). Except for the granite sample, the Th/ U (0.12 ~ 1.23) of the older core of all other samples are all larger than 0.1, which indicate a magmatic origin. The younger rim, however, showed Th/ U (0.0013 ~ 0.0098) < 0.01 indicating a metamorphic origin. The Th/ U are all larger than 0.1 for the granite sample indicating the granite crystallized during the 618 Ma (concordant age) Pan-African Orogeny, and the few older cores are inherited zircons. In order to decipher the tectonic setting of the magmatic sources, IV.

(6) whole rock XRF analysis of major and trace element with Hf isotope analysis of zircons are also conducted. Although the zircon U-Pb geochronology data of the Paleoproterozic core suggested a magmatic origin, the whole rock trace element distributions are far below the normal magmatic rocks. Which pointed out that the gneiss and mylonite samples are metasedimentary rocks, and the age in the core of zircons are inherited. Since the age in the core of zircons corresponds to the Paleoproterozoic magmatic event formed during the Eburnean orogeny by the collision between the South America and West Africa. This could be the source of sediments for these metasedimentary rocks. Although the sedimentary age was uncertain, it is possible that they have the same age as the Mesoproterozoic Yaoundé series in the Lom basins. The wide range of εHf (T) values (－18.5 ~ －11.2; +1.0 ~ +10.5) of the 618 Ma granite sample suggested the formation of this magmatic event is probably due to the Pan-African orogeny with collision between the Eastern Sahara Block with the Congo Craton and West African Craton under an arc environment. Other than magmatic event, the Pan-African orogeny also generated the regional metamorphism of surrounding metasedimentary rocks. V.

(7) Petrological analysis of the metasedimentary rocks showed retrograde metamorphism from amphibolite facies to greenschist facies after the Pan-African event. 40Ar/39Ar dating of metamorphosed K-feldspar mineral separates of 10CC02C sample showed age ranges from 589 ~ 519 Ma indicating the deformation event of the TBSZ had occurred under the lower greenschist facies during this time. Key words: U-Pb Zircon Dating, Tcholliré-Banyo Shear Zone (TBSZ), Tectonic evolution, Pan-African. VI.

(8) Contents 摘要............................................................................................................. I Abstract ................................................................................................... IV I.. Introduction .........................................................................................1. II.. Geological setting .............................................................................12. III.. Analytical Methods ...................................................................19 1.. Zircon U-Pb geochronology .....................................................19. 2.. Hf isotope analyses ...................................................................20. 3.. 40. 4.. Geochemical and Petrological analysis ....................................22. IV.. Ar/39Ar step-heating method ..................................................21. Results .......................................................................................23 1.. Zircon U-Pb geochronology analysis .......................................23 1.1.. TBSZ sample 10CC02 ..................................................23. 1.2.. TBSZ sample 10CC03 ..................................................29. 1.3.. CASZ sample 10CC04 ..................................................35. 1.4.. CASZ sample 10CC05 ..................................................37. 2.. Geochemistry analyses .............................................................39. 3.. 40. 4.. Zircon Hf isotope analysis ........................................................47. Ar/39Ar step heating analysis .................................................43. VII.

(9) 5. V.. Petrological Analyses................................................................50. Interpretation .....................................................................................78 1.. Geochronology..........................................................................78. 2.. Geochemistry and petrography .................................................79. VI.. Discussion .................................................................................82 1.. Reconstruction tectonic evolution ............................................82 1.1.. 2.. Possible protolith Eburnean sedimentary ......................82. Syn-Pan-African metamorphism and magmatism....................83. VII.. Conclusions ...............................................................................87. VIII.. Reference ..................................................................................88. VIII.

(10) List of Figures Fig. 1. Reconstruction of the western Gondwana at Permo~triassic time (~250Ma) ....................................................................................................3 Fig. 2. Gravity diagram showing the location of CASZ and TBSZ, modified from .............................................................................................6 Fig. 3. Location map of WCARS ...............................................................6 Fig. 4. Reconstruction of South America and Africa from early Cretaceous to late Cretaceous. ..................................................................10 Fig. 5. Geological map of North Cameroon with previous geochronology results ........................................................................................................14 Fig. 6. Geological map of North Cameroon showing the sample location of study area. .............................................................................................16 Fig. 7. Zircon U–Pb Concordia diagram with selected CL electron micrographs for hornblende plagioclase gneisses metamorphic layering (10CC02CV) .............................................................................................25 Fig. 8. Zircon U–Pb Concordia diagram with selected CL electron micrographs for hornblende plagioclase gneisses (10CC02CM) .............27 Fig. 9. Zircon U–Pb Concordia diagram with selected CL electron micrographs for deformed feldspar vein (10CC03A)...............................31 IX.

(11) Fig. 10. Zircon U–Pb Concordia diagram with selected CL electron micrographs for mylonite (10CC03C) ......................................................33 Fig. 11. Zircon U–Pb Concordia diagram with selected CL electron micrographs for granite (10CC04)............................................................36 Fig. 13. ACF diagram of TBSZ samples ..................................................40 Fig. 14 Normalized element distributions ................................................42 Fig. 15. Microphotographs of variably foliated gneiss.............................45 Fig. 16. Plot of εHf (T) values with error bar verses U-Pb age of zircons form TBSZ and CASZ samples. ...............................................................49 Fig. 17. The metamorphic facies of hornblendes and amphibole classification of sample 10CC02A. ..........................................................55 Fig. 18. The metamorphic facies of hornblendes and amphibole classification of sample 10CC02B............................................................58 Fig. 19. The metamorphic facies of hornblendes and amphibole classification of sample 10CC02C............................................................61 Fig. 20. The metamorphic facies of hornblendes and amphibole classification of sample 10CC02D. ..........................................................64 Fig. 21. The metamorphic facies of hornblendes and amphibole classification of sample 10CC02E. ...........................................................67 X.

(12) Fig. 22. The metamorphic facies of hornblendes and amphibole classification of sample 10CC03A. ..........................................................70 Fig. 23. The metamorphic facies of hornblendes and amphibole classification of sample 10CC03B............................................................73 Fig. 24. The metamorphic facies of hornblendes and amphibole classification of sample 10CC03C............................................................73 Fig. 25. The ACF diagram and SEM image. ............................................81 Fig. 26. (a) Relative probability diagram of zircon U–Pb ages of the core of zircons. (b)The geothermal chronology evolution of TBSZ and CASZ ...................................................................................................................86. XI.

(13) Tables of Contents Table 1. Sample list ...................................................................................93 Table 2. Zircon U-Pb age data from the hornblende gniess (10CC02C), phyllonitic feldspar vein (10CC03A), mylonite (10CC03C), granite (10CC04) and gniess (10CC05) from the TBSZ and CASZ ....................94 Table 3. Whole-rock major element compositions for the TBSZ and CASZ samples. .......................................................................................104 Table 4. Whole-rock trace element compositions for the TBSZ and CASZ samples. ...................................................................................................105 Table 5. Analytical data of K-feldspar separates ....................................108 Table 6. LA-MC-ICPMS Hf isotope data of zircons from the TBSZ and CASZ ...................................................................................................... 110 Table 7. Electron probe micro analytical data of amphiboles from TBSZ. (Calculated from Amp IMA97; Hawthorne et al. (1997) ....................... 116 Table 8. Analytical data of Hornblende, Biotite, Chlorite, Plagioclase, Albite and Epidote of sample 10CC02B and 10CC03A ........................123. XII.

(14) I.. Introduction During the Proterozoic, there were several supercontinents, or major. groupings of continents. At these times there was a noticeable lack of orogenic activity. However, subduction giving rise to island arcs, Andean-type arcs and accretionary orogens can take place on the outer boundary of a supercontinent. The tectonic activity just started at Proterozoic and much different from present day; the early Proterozoic mantle was hotter than that of today lead to the production of thinner oceanic plates and the subduction of younger plates, leading a greater proportion of Andean-type continental arcs than on the present Earth (Windley, 1984). A large group of cratons were stabilized at ~2100-2000 Ma in West Africa and northeastern South America. Cratons in both continents appear to have gone through the Trans-Amazonian orogeny in South America and Eburnean orogeny in Africa (Rogers and Santosh, 2003). At this time, the São Francisco Craton of Brazil and the Congo Craton of Africa, forming a single continental block connected South America and West Africa, called Atlantica (Fig. 1) (Alkmim et al., 2006; Rogers and Santosh, 2003; Zhao et al., 2002b). During the Eburnean– Transamazonian orogeny at ca. 2.1 Ga, the collision of South America 1.

(15) and West Africa block formed the central African fold belt (CAFB). In the CAFB of northern Cameroon and southeastern Nigeria, U–Pb zircon ages on plutonic rocks permit constraint of collisional and post-collisional events within the age range 640–570 Ma. The region north of the Congo Craton in Cameroon was primarily the locus of two successive orogenies, Eburnean at 2100 Ma and Pan African at 600 Ma (Fig. 1) (Toteu et al., 2001). The Tcholliré region has benefited from previous geochronological studies, mostly U–Pb and Sm–Nd ages. These data indicate the presence of Paleoproterozoic rocks reworked during the Pan-African events (Bouyo Houketchang et al., 2009). During Paleo-Mesoproterozoic Columbia supercontinent, TBSZ located at the middle of Congo Craton, Eastern Sahara Block and East African Craton, in order to resolve the tectonic evolution, one must find samples that recorded the completed evolution.. 2.

(16) Fiig. 1. Recoonstruction n of the w western Go ondwana at a Permo~ttriassic tim me (~ ~250Ma) modified m from f Moullin et al. (2 2010); Atlantica craaton symb bols (ooblique linne zone) modified m frrom Rogerrs and San ntosh (20003).. 3.

(17) There is a great relationship between the formation of CAFB and Central African Shear Zone (CASZ). CASZ is a North-East direction, about 4000 km long shear zone, from Gulf of Guinea pass through Cameroon and Chad to Sudan (Ibrahim et al., 1996). At northern Cameroon, CASZ divided into Central Cameroon Shear Zone (CCSZ) and Tcholliré-Banyo Shear Zone (TBSZ); CCSZ strikes N70°E, TBSZ directed N50°E (Fig. 2). The Tcholliré region belongs to the NW part of the Adamawa–Yadé Domain, while the Banyo region is part of the West-Cameroon Domain (Toteu et al., 2004). TBSZ located between CASZ and the Benue trough, and intently related to the activity histories of CASZ and rift basins. From Bouguer gravity anomalies figure (Fig. 2), CASZ is the rather low gravity area, which take shape into a main liner structure in West and Central African rift system (WCARS, Fig. 3), where contains many pull-apart basins and narrow rifts (Lu et al., 2009). The origin of WCARS is generally attributed to the breakup of Gondwana during Pan-African orogeny, and the opening of the South Atlantic Ocean and Indian Ocean, starting at about 130 Ma. WCARS is divided into the West African subsystem (WAS) and the Central African rift subsystems (CAS), and the WAS-CAS basins contain a large volume of Cretaceous 4.

(18) sediments (Genik, 1993) (Fig. 3).. 5.

(19) Figg. 2. Graviity diagram m showingg the locattion of CA ASZ and T TBSZ, modified from m (Smith and Sandw well, 1997 7).. Figg. 3. Locattion map of o WCARS S (obliquee line zonee), modifieed from 6.

(20) Genik (1993).. 7.

(21) From previous study, due to TBSZ was situated among various ancient cratons, and experimented several complex tectonic events, the formation of TBSZ is a controversial topic. According to previous geochronology results around 2 Ga, most of the dating age is given from both meta-igneous and metasedimentary rocks, we decipher the rock type of protolith and built the later evolution of them. Furthermore, most of the outcrops at CASZ was overburdened under the desert, the better outcrop only located at Cameroon region, so it is difficult to understanding the activity history of CASZ. There are two fractions of explanation about this region: Ngako et al. (2008) suggested that the activities of CASZ was related to Pan-African collision event between 640 and 580 Ma. But Fairhead et al. (1991) brought out another model that the opening of WCARS drive the activity of CASZ. The early breakup phase of the South Atlantic Ocean prior to the opening of the Equatorial Atlantic Ocean at about 130 Ma, and was the time WCARS began to develop, resulting in differential movement between west and southern Africa. The breakup of Equatorial Atlantic Ocean took place at 119 Ma and continued until the end of Cretaceous about 105 Ma (Fig. 4). Thus, we suppose that the evolution of TBSZ can be traced back from Paleo-Mesoproterozoic 8.

(22) supercontinent resulted from the collisional event of Eburnean orogeny during 2.1-1.8 Ga, to western Gondwana tectonic evolution during Pan African until the opening of Atlantic at Cretaceous (Fig. 1 and 4).. 9.

(23) Figg. 4. Reconnstruction of South A America and a Africaa from earrly Creetaceous too late Crettaceous. M Model showing the broke b up oof Atlanticc at Creetaceous, modified m from f Fairhhead and Green G (1989).. 10.

(24) The Adamawa-Yadé Domain and West-Cameroon Domain is poorly surveyed, particularly with regard to geochemical and geochronological studies. Data exist only for a few isolated massifs, which probably belong to another batholith in south-western Cameroon. In this paper, based on field, geochronological, geochemistry, and petrographical results, we present and discuss a proposed reconstruction model for the formation and evolution history of TBSZ.. 11.

(25) II. Geological setting Most of the Precambrian basement of Cameroon belongs to the Pan-African belt of central Africa, or Neoproterozoic CAFB, that stretches along the northern border of the Congo Craton (Njanko et al., 2006), and the Paleoproterozoic basement (2030–2200 Ma) forms the northern margin of the WAC (Abati et al., 2010). The CAFB can be summarized as the result of the convergence and collision between the São Francisco – Congo Cratons (SFCC) and the WAC and a Pan-African mobile domain (Toteu et al., 2004). This collision event started the activity of CASZ and TBSZ, which in north Cameroon is a branch of the NE-SW trending CASZ, these shear zones continue into NE Brazil, where they control the emplacement of Pan-African granotoids (Neves and Mariano, 1999). Pan-African granitoids belong to Neoproterozoic Batié pluton, which is an elongated body, the long axis trends NNE-SSW, and foliation striking parallel to the CCSZ. TBSZ belongs to a complex batholith referred to Adamawa-Yadé batholith, which represents a Paleoproterozoic basement that was dismembered during the Pan-African orogeny (Toteu et al., 2004). Pan-African pre-, syn-, to late-tectonic granitoids (diorites, granodiorites, and granites) mainly of calk-alkaline 12.

(26) composition emplaced between 660 and 580 Ma, and these granitoids were transformed into orthogneisses during the first two phases of deformation, syn- to late-tectonic porphyritic hornblende-biotite granitoids whose emplacement was controlled by transcurrent shear zones, and post-tectonic high-level alkaline granitoids (Fig. 5). Emplacement of plutonic rocks is recorded all along this tectonic evolution.. 13.

(27) Figg. 5. Geoloogical map p of Northh Cameroo on with preevious geoochronolo ogy resuults (Bouyyo Houkettchang et aal., 2009; Njiekak et e al., 20088b; Tanko et al., 200 Njiiosseu et al., a 2005; Tchameni T 06; Toteu et al., 20001).. 14.

(28) The Nagoundéré Pan-African granitoids in Central North Cameroon belong to a regional-scale massif, which is referred to as the Adamawa-Yadé batholith. The granites were emplaced into a ca. 2.1 Ga remobilized basement composed of metasedimentary and meta-igneous rocks that later underwent medium- to high-grade Pan-African metamorphism. The Tcholliré region consists of poly-deformed biotite-amphibole gneisses and orthogneisses with minor occurrences of metapelites and garnet amphibolites. The Adamawa-Yadé batholith consists of a great variety of more or less deformed rock-types of different ages. The various lithologies of the batholith include: dominant biotite and amphibole granites, biotite granites, biotite, amphibole and pyroxene granites, biotite and muscovite granites, diorites, gabbros, and syenites (Fig. 5) (Tchameni et al., 2006). Previous geochronology results showed that two major sets of ages can be noted: one is the older ages ~2000 Ma of Adamawa-Yadé domain, but it is uncertain that this pluton representing a magmatic event or just metasediments. Another younger age group of granite ~600 Ma, representing both magmatic and metamorphic event accompany Pan-African orogeny. Thus, we collected the two different rock types to decipher the tectonic significance. 15.

(29) Figg. 6. Geoloogical map p of Northh Cameroo on showing g the sampple locatio on of sstudy areaa. Red linee: TBSZ, bblue line: mylonite m zone z and bblack line: CA ASZ (Kankkeu et al., 2009; Nji ekak et al., 2008a; Penaye P et al., 2006; Totteu et al., 2004) 2 .. 16.

(30) There are existences of high-temperature metamorphic rocks due to shearing heating in syn-shearing granite. In order to understanding the metamorphic conditions, micropetrography helps to recognize the mineral assemblage and infer temperature of metamorphism. With observation of new field and microstructural data and strain analysis, the tectonic history of TBSZ can be devided into at least three events (Wu, 2012). The earlier event Dn-1 formed recumbent folds with NE-SW striking, SE dipping fold axial planes. The mineral assemblage of Sn-1 is hornblend ± garnet ± biotite ± Albite suggested that P-T conditions in under middle Amphibolite facies. Dn-1 is result from the amalgamation of the Congo craton and the Eastern Saharan Block in N-S direction by the analysis of structure and strain. Dn event formed tight upright folds with NE-SW striking subvertical axial planes and parallel to CASZ. Sn with mineral assemblage Biotite ± Chlorite ± Sphene ± K-feldspar form under greenschist facies and left-lateral shearing. By reconstruction of strain ellipsoids, the main stress direction is NNW-SSE to NW-SE during Dn with transtension strain pattern . Dn+1 is brittle deformation close to surface by low temperature (Wu, 2012).. 17.

(31) We collected 10 samples from 4 outcrops at north Cameroon, 7 samples located at TBSZ including 2 hornblende plagioclase gneisses, 1 cataclasite, 1 garnet hornblende gneiss, 3 mylonites and 1 deformed feldspar vein in mylonite (Table 1). Hornblende plagioclase gneisses show apparent stripes, and charcoal grey amphibole alternate with white feldspar and quartz belt; one of the hornblende plagioclase gneisses contain garnet about 0.5 cm. Cataclasite contains amphibole and phenocrystic feldspar, and was cut though by quartz vein. Mylonite with white feldspar zone and dark amphibole zone cross lamination. Feldspar and amphibole grain were broken into small grain size in mylonite, and feldspar vein showed light pink. Two samples are situated at south CASZ, one is deformed granite, the other is gneiss, which is also composed by amphibole, quartz and pink feldspar, with average grain size, but unapparent stripes (Fig. 6).. 18.

(32) III. Analytical Methods 1. Zircon U-Pb geochronology Zircons separated from sample 10CC02C, 10CC3A, 10CC03C, 10CC04 and 10CC05 are conducted by the normal procedure of heavy liquid and wobbly table. The separated zircons were mounted in epoxy and photographed in backscatter and cathodoluminescence (CL) imagery. Most of the zircon crystals are subhedral with oscillatory zonation, and prismatic in shape around 150~200 μm in size. We also use Hf isotope analysis to evaluate crustal residence and growth because of the considerable amount of high field strength elements (HFSE) within zircon (Hawkesworth and Kemp, 2006). The combination of U-Pb dating with Hf isotope analysis of zircons reveals the crystalline ages and the relative contributions of juvenile (directly mantle derived) crust versus recycled continental crust (Scherer et al., 2007). Zircon U–Pb isotopic analyses were performed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at National Taiwan University in Taipei. The full set-up and methods are described by (Chiu et al., 2009). The laser ablation was performed using a He gas carrier to improve material transport efficiency. Standard blanks were measured for ∼1 min and 19.

(33) calibration was performed using GJ-1 zircon standard, Nancy–Harvard reference zircon 91500 and Australian Mud Tank carbonitite zircon. Data processing was completed using GLITTER 4.0 for the U–Th–Pb isotope ratios and common Pb. Isoplot v. 3.0 was used to plot the Concordia diagram and to calculate the weighted mean U–Pb age (Ludwig, 2003). The standard GJ-1 zircon, which yielded 207Pb/206Pb age of 608.5 ± 0.4 Ma (2σ) and an intercept age of 608.5 ± 1.5 Ma (2σ) using isotope-dilution thermal ionization mass spectrometry (ID-TIMS) (Jackson et al., 2004). 2. Hf isotope analyses 176. Hf/177Hf isotopic ratio analysis was conducted at the exact same. spots for age dating utilizing the Neptune MC-ICP-MS with a larger beam diameter (60 μm) and a laser repetition rate of 10 Hz at 100 mJ. The detailed analytical method was described in (Wu et al., 2006). During the analysis, the results of 176Hf/177Hf for Mud Tank Hf standard solution in a long term give an average 176Hf/177Hf ratio of 0.282535 ± 45 (n = 140, 2SD) normalized to 179Hf/177Hf = 0.7325 using an exponential law for mass bias correction (Wu et al., 2006). The εHf(T) values from these 20.

(34) magmatic zircons of the Khao Tao gneiss scatter from +20.7 to −43.2, corresponding to (TDMC) model ages of between 501 Ma and 2419 Ma. 3.. 40. Ar/39Ar step-heating method. K-feldspar separated from sample 10CC02C, and these samples were also examined for 40Ar/39Ar dating by step-heating method using a low-blank furnace. The disk was rinsed for 5 minutes in deionized water and 5 minutes in acetone repeatedly for 3 rounds within the Ultrasonic vibrator. Rock disk and standard LP-6 biotite (Odin et al., 1982) were irradiated for 30 hours in McMaster Nuclear Reactor in Canada, and decayed for more than 1 month before 40Ar/39Ar experiments. The J values were calculated from the argon compositions of the LP-6 biotites with a 40Ar/39Ar age of 128.4 ± 0.2 Ma (Lo et al., 2002b). Sample and monitor mineral were degassed using a Nd/YAG laser operated in continuous mode after the irradiation. Sample preparation and analytical procedures followed those outlined by Lo and Lee (1994) and Lo et al. (2002a). Corrections of isotope compositions according to mass discrimination, system blanks, isotope interference and radiometric decay were carried out before 40Ar/39Ar age calculation (see Lo et al. (2002a); 21.

(35) Lo and Lee (1994) for details). 4. Geochemical and Petrological analysis All rock samples and enclaves were collected for petrographic and geochemical studies. Mineral compositions were determined using a Cameca SX-50 electron microprobe. Quantitative analysis of mineral chemistry was performed by a field-emission type electron micro probe (JEOL EPMA JXA-8500 F) with wave-length dispersive spectrometers (WDS) at the Institute of Earth Sciences, Academia Sinica, Taipei. Major oxides were determined by wavelength-dispersive X-ray fluorescence spectrometry (PANalytical AXIOSmax) and Fluxer and Manufacture Classe at National Normal Taiwan University, Taipei. Electronic balance and USGS standard geological reference materials of carbonatite, dunite, basalt, diorite and shale. Trace elements were carried out at ACTLABS (Ancaster, Ontario, Canada). More information on the procedure, precision and accuracy of ACTLABS ICP-MS analyses can be found at http://www.actlabs.com/.. 22.

(36) IV. Results The zircon U–Pb data are listed in the Table 2; whole rock compositions are given in Table 3 and 4; 40Ar/39Ar data listed in Table 5. Lu–Hf isotopic listed in the Table 6. Electron probe micro analytical data of amphiboles from TBSZ listed in the Table 7. Petrology chemical composition data list in Table 8. 1. Zircon U-Pb geochronology analysis The zircon U-Pb age result are summarized in Appendix Table 2 and plotted using Concordia diagrams and Th/U ratio diagram in Fig 7 to 12, respectively, together with Th/U ratios, indicating the distinguishing zircons of igneous and metamorphic origin (Rubatto, 2002). The result include 206Pb/238U ages of the six samples, based on a total of 167 individual zircon analyses. 1.1. TBSZ sample 10CC02 U-Pb dating for zircons from hornblende plagioclase gneisses 10CC02C divided into two groups: feldsic vein of metamorphic layering (10CC02CV) and matrix (10CC02CM), both of them show oscillatory zoning from center to margin with some signs of alteration in CL images, 23.

(37) preserving inherited core and metamorphic zonation with low-bright (dark) core and a wide high-bright rim. In sample 10CC02V, the result obtain 21 spots on 20 grains, isotopic data give concordant ages on 607 ± 37 Ma (MSWD = 0.13) at metamorphic rim (Fig. 7). But the older age group not that concordant on Concordia diagram, age range from 1927 Ma to 2133 Ma. In sample 10CC02M, the result obtain 43 spots on 32 grains, and isotopic data give concordant age on 596 ± 17 Ma (MSWD = 0.069) on metamorphic rim (Fig. 8), and older age range from 1924 Ma to 2138 Ma. Three zircon grains contain both cores and rims, with Paleoproterozoic cores of 2113, 1992, 1962 and 2081 Ma and Pan-African rims of 616, 694, 604 and 586 Ma, respectively (Fig. 7, 8). Otherwise, the Th/U ratio indicate that the Th/U ratio of magmatic core is higher than 0.1; the Th/U ratio of metamorphic rim is lower than 0.01 (Rubatto and Gebauer, 2000). The Th/U ratio of most inherited core > 0.1, and metamorphic rim < 0.01, indicating both magmatic core and metamorphic rim preserved in sample 10CC02. Which is interpreted to represent that the rim ages indicated the recrystallization age of a metamorphic event during Pan-African orogeny, but it is uncertain the crystallization age of the zircons. 24.

(38) Figg. 7. Zirconn U–Pb Concordia ddiagram with w selectted CL eleectron miccrographs for hornb blende plaggioclase gneisses g metamorph m hic layering g (100CC02CV)), most zirrcon grainns display interrupteed oscillatoory zoning g 25.

(39) from center to margin, and some grains have inherited cores. The red and green circles indicate the positions of LA-ICP-MS U–Pb dating analysis. Concordant ages are indicated in green, and inherited cores are shown in red. Concordant ages are indicated in green, and inherited cores are shown in red. 607 ± 37 Ma with MSWD = 0.13 for average rim age were calculated.. 26.

(40) Fiig. 8. Zircoon U–Pb Concordia C a diagram with seleccted CL ellectron m micrographhs for horn nblende plaagioclase gneisses (10CC02C ( CM), mostt zirrcon grainns display interrupteed oscillattory zoning from cennter to 27.

(41) margin, and some grains have inherited cores. The red and green circles indicate the positions of LA-ICP-MS U–Pb dating analysis. Concordant ages are indicated in green, and inherited cores are shown in red. 596 ± 17 Ma with MSWD = 0.069 for average rim age were calculated.. 28.

(42) 1.2. TBSZ sample 10CC03 Zircons separated from deformed feldspar vein in mylonite 10CC03A also show oscillatory zoning from center to margin with some signs of alteration in CL images, preserving inherited core and metamorphic zonation, darker core and a narrow high-bright rim. The result obtain 40 spots on 31 grains, isotopic data give concordant ages on 610 ± 34 Ma (MSWD = 0.17) at metamorphic rim (Fig. 9). The older age group display detrital zircon signature of discordance on Concordia diagram, and age range from 1413 Ma to 2140 Ma. Zircons separated from mylonite 10CC03C also show oscillatory zoning from center to margin with signs of alteration in CL images, and preserve inherited core and metamorphic zonation, darker core and a wide high-bright rim. The result obtain 39 spots on 31 grains, isotopic data give age on 600 ± 26 Ma (MSWD = 0.26) at metamorphic rim (Fig. 10) with older age range from 1842 Ma to 2098 Ma. Five zircon grains contain both cores and rims, with older cores of 1904, 1946, 1930, 1909 and 1906 Ma and younger rims of 631, 556, 604, 696 and 657 Ma, respectively (Fig. 9, 10). Thus, the result also interpreted as the age of metamorphic rim represented the recrystallization age of Pan-African metamorphism, but the age of zircons 29.

(43) crystallized is unclear.. 30.

(44) Fiig. 9. Zircoon U–Pb Concordia C a diagram with seleccted CL ellectron m micrographhs for deformed felddspar vein (10CC03A A), most zzircon graains 31.

(45) display slight interrupted oscillatory zoning from center to margin, and some grains have inherited cores. The red and green circles indicate the positions of LA-ICP-MS U–Pb dating analysis. Concordant ages are indicated in green, and inherited cores are shown in red. An average peripheral rim age 610 ± 34 Ma with MSWD = 0.17 were calculated.. 32.

(46) Fiig. 10. Zircon U–Pb b Concordiia diagram m with seleected CL eelectron m micrographhs for mylo onite (10C CC03C), most m zircon n grains di display slig ght innterrupted oscillatory y zoning ffrom centeer to marg gin, and soome grainss haave inheritted cores. The red aand green circles c ind dicate the ppositions of 33.

(47) LA-ICP-MS U–Pb dating analysis. Concordant ages are indicated in green, and inherited cores are shown in red. 600 ± 26 Ma with MSWD = 0.26 for average rim age were calculated.. 34.

(48) 1.3. CASZ sample 10CC04 Zircons from granite sample 10CC04 are darker than others in CL image, and they show well-developed magmatic zoning with darker core and slight bright rim. The results obtained 22 spots on 22 grains define a concordance group on the Concordia diagram with an age of 618 ± 6 Ma (MSWD = 0.95), only a 2113 Ma age (Fig. 11). Both cores and rims have Th/U ratio higher than 0.1, which are characteristic of magmatic zircons, and thus implying the magmatic origin of Pan-African orogeny.. 35.

(49) Figg. 11. Zircoon U–Pb Concordia C a diagram with seleccted CL ellectron miccrographs for granitte (10CC004), most zircon z graiins displayy slight inteerrupted oscillatory o zoning fro rom centerr to margin n, but few w grains haave inhherited corres. The reed and greeen circles indicate the positioons of LA A-ICP-MS U–Pb datting analyssis. Conco ordant agees are indiccated in greeen, and innherited co ores are shhown in reed. An rim m age of 6118 ± 6 Maa witth MSWD D = 0.95 fo or average age were calculated d.. 36.

(50) 1.4. CASZ sample 10CC05 In Th/U ratio, all the plots are higher than 0.1 in gneiss sample of 10CC05; thirty plots were investigated for dating. The results obtained on 30 grains display a pattern of detrital zircon, and define a discordance line on the Concordia diagram with an upper intercept age of 2455 ± 73 Ma and a lower age of 638 ± 88 Ma (MSWD=13) (Fig. 12). The presence of discordant points below the Concordia curve may have resulted from Pb loss. The age of inherited zircons suggest that this gneiss sample had experimented sedimentary event at about 1800 Ma.. 37.

(51) Figg. 12. Zircoon U–Pb Concordia C a diagram with seleccted CL ellectron miccrographs for gneisss (10CC055), most ziircon grain ns displayy slight inteerrupted oscillatory o zoning fro rom centerr to margin n, and inhherited cores. Thee red and green g circcles indicaate the positions of LA-ICP-M L MS U–Pb datiing analyssis. Conco ordant ages are indiccated in grreen, and iinherited corres are shoown in red d. Thirty d ata-spots on o an line define ann upper inteercept agee of 2455 ± 73 Ma aand a loweer intercept age of 6338 ± 88 Ma M (MSWD = 133).. 38.

(52) 2. Geochemistry analyses 2.1. Major elements In ACF diagram, except CASZ sample (10CC05), TBSZ samples plot on quartzo-feldspathic field, indicating the protolith of TBSZ is sandstone (Eskola, 1915). In terms of major elements, some features are common to pluton of TBSZ: (1) the rocks are calc-alkaline associations at continental crust field (Fig. 13); (2) silica contents typically > 50 wt. %. To sum up, ACF diagram pointed out they are sandstone, but we need more accurate geochemistry analyses to clarify the rocks type of TBSZ.. 39.

(53) Figg. 13. ACF F diagram of TBSZ samples (E Eskola, 19 915).. 40.

(54) 2.2. Trace elements In rare earth elements (REEs) analyses, TBSZ rocks are characterized by significant deficiency of light rare earth elements (LREEs) and variable depletion in heavy rare earth elements (HREEs), and most of the HREEs are under detection limit (Table 4). Data also show low content in large ion lithophile elements (LILEs; e.g., Cs, Rb, Ba, Sr) and depletion of high field strength elements (HFSEs; e.g., Nb, Ta, Ti, Hf) (Table 4).The primitive mantle-normalized rare earth element patterns (Fig. 14a, the normalizing values are taken from Sun and McDonough (1989) show a decreasing trend, and slight negative anomaly in Ta with positive anomaly on Ti, strongly negative anomaly of Y. The north America shale composite-normalized rare earth element patterns (Fig. 14b the normalizing values are taken from Haskin et al. (1968) show positive anomaly on Ba, Ce, Ti and Y. These characteristics are considered mafic to intermediate rocks, and typical of crustal-derived magmas. It is the strongest evidence that the protolith of TBSZ is metasediments.. 41.

(55) (a). (b)). Figg. 14 Norm malized eleement disttributions in (a) prim mitive manntle (Sun and MccDonough, 1989), (b b) north A America sh hale compo osite (Hasskin et al., 19668). 42.

(56) 3.. 40. Ar/39Ar step heating analysis. In Sample 10CC02C taken from TBSZ, the albitization of K-feldspar can be identified via micro petrography of SEM and CL imagery analysis (Fig. 15). A comparison of the major element chemistry of K-feldspar and albite are showed a contrary relationship between Na and K cation content (Table 5). With many albite grains replaced K-feldspars producing intrusive shapes that embay the K-feldspar porphyroblasts along their edges (Figs.15a, b, c), the textures and this pattern present in these feldspathic rocks are most easily explained by local hydrothermal, metasomatic reactions in a partially open system. This suggests the precipitation of K-feldspar metamorphic layering from hydrothermal fluids at lower temperatures during deformation via incomplete replacement reaction of K-feldspar by albite by the open system ion exchange reaction: K-feldspar + Na+ = Albite + K+. 40. Ar/39Ar isotopic analyses of the K-feldspar of hornblende. plagioclase gneiss (10CC02C) yielded flat age spectra with identical ages of 519.58 ± 7.45 and 589.22 ± 1.54 Ma, respectively. The K-feldspar showed complex mixing asymptote spectra with younger ages ranging 43.

(57) from 513.9-525.3 Ma at lower temperature steps, that climbed to older ages ranging from 584.3-594.2 Ma for higher temperature steps (Fig. 15d). This kind of apparent age profiles indicated the ages are obtained by mixing gas from two (one retentive and one less-retentive) different microstructural reservoirs (Forster and Lister, 2004). Since these K-feldspar porphyroblasts co-exiting with hornblende reveal a continuous thermal history in the temperature rang 150～350℃. Thus we correlated the older ages (~589 Ma) as the higher bound (300-250℃) of the cooling ages, and the younger ages (~519 Ma) as the lower bound (250-150℃) of the cooling ages, that might partially reset by later deformation (Fig. 15d).. 44.

(58) Kporphy. Kalter. Abb. Pl. Figg. 15. Micrrophotograaphs of vaariably folliated gneiiss. (a) SE EM image shoowing a tyype porphy yroblast off partially albitized (Ab) K-feeldspar (K K) wraapped by folia f of rellic biotite and quarttz (Qtz) sh howing a bbounding struucture of K-feldspar K r. Fractureed and extended K-ffeldspar w with crackss filleed by albiite and quaartz is pressent on the edge of K-feldspaar. (b) CL imaage showing the gneeiss is com mposed mo ostly of brright blue fractured mettamorphicc layering K-feldspaar relics (K Kporphyro), which w are surroundeed by grey blue color for the albite (Ab) wheere dark bllue color neoocrystallizzed K-feldspar (Kalteer) precipittated to fill the crackks. 45. Qtz.

(59) Plagioclase (Pl) is yellow, and Quartz (Qtz) is black. (d) 40Ar/39Ar age spectrum of K-feldspar separates from gneiss sample 10CC02C with steps climbing from ~589 to ~519 Ma. It is interpreted to reflect a mixture of cooling ages.. 46.

(60) 4. Zircon Hf isotope analysis Zircon Hf isotope data are shown by plotting εHf (T) values vs. U– Pb ages of total 129 dated zircon grains from six samples, 10CC02CV, 10CC02CM, 10CC03A, 10CC03C from TBSZ, and 10CC04 and 10CC05 at south of CASZ. The results are listed in Table 6 and plotted in Fig. 16. In sample 10CC02CV and 10CC02CM, Paleoproterozoic (2042 ± 22 Ma and 2061 ± 100 Ma) inherited zircon cores have roughly positive εHf (T) value with a little negative value from −0.4 to +11.9 and Hf model ages TDM2 = 2.0–2.7 Ga (TDM1 of 2.0–2.5 Ga). The rims, which gave a mean Pan-African age of 607 ± 37 Ma and 596 ± 17 Ma, in contrast, have negative εHf (T) value of −6.0 to −24.7 and Hf model ages TDM2 = 2.0– 3.1 Ga (TDM1 of 1.4–2.1 Ga). Zircons from 10CC03A and 10CC03C, with average age of 1983 ± 130 Ma and 2009 ± 87 Ma have approach zero εHf(T) values ranging from −4.5 to +4.2 for inherited cores (TDM1 = 1.5–2.6 Ga, TDM2 = 1.4–2.9 Ga), and negative εHf (T) value from −17.9 to −34.1 for Pan-African age rims (TDM1 = 1.9–2.5 Ga, TDM2 = 2.7– 3.8 Ga). It is evident that the Hf isotopic ratios are higher in the zircon cores than the rims, and the εHf (T) values become smaller with the decreasing of age. From the wide range of εHf (T) values (-4.5~11.9) of 47.

(61) sample 10CC02 and 10CC03, we cannot confirm that the Paleoproterozoic core crystallized from depleted mantle with juvenile source, but the negative εHf (T) value of Neoproterozoic rim most suggesting recycling of ancient crust material. The Pan-African concordant zircons from 10CC04 divided into two groups, with εHf (T) values from +1.0 to +10.5 (TDM1 = 0.9–2.1 Ga, TDM2 = 1.1–2.1 Ga) of core ages, and the negative one from −11.2 to −18.5 (TDM1 = 1.6–1.9 Ga, TDM2 = 2.3–2.5 Ga) of rim ages. The difference of εHf (T) values indicate the group with positive εHf (T) values caused from magmatic event, the other negative one was continental environment, and they were crystallized at the same age. The mixing εHf (T) values indicating there was an arc environment. All the Paleoproterozoic discordant zircons from 10CC05 exhibit negative εHf (T) values from –4.2 to –43.2 (TDM1 = 2.7–3.0 Ga, TDM2 = 3.1–4.3 Ga). The negative εHf (T) value on 10CC05 observed exhibit a decrease trend with U-Pb age, which is the characteristic of detrital zircon.. 48.

(62) Figg. 16. Plot of εHf (T) values w with error bar b versess U-Pb agee of zircon ns form m TBSZ and a CASZ Z samples... 49.

(63) 5. Petrological Analyses Hornblende is the most abundant mineral in TBSZ rocks, and appears in every structure domain, so we use electron probe micro analyzers to detect chemical composition of hornblendes on samples 10CC02A to E and 10CC03A to C (Table 7). In order to constrain the temperature and pressure of different amphibole domain, we use Ti, Al and Si content to estimate temperature-pressure conditions (Raase, 1974). A comparison of the composition of hornblende from rocks of different metamorphic grade, is displayed by Ti content. In Fig. 17a~24a the Ti content of hornblendes from rocks of different metamorphic facies is represented. The following metamorphic facies have been distinguished: 1. Greenschist-amphibolite transition facies, characterized by epidote-amphibolites which may contain albite or albite besides oligoclase. 2. Low-grade amphibolite facies, with amphibolites containing oligoclase-andesine and politic rocks containing staurolite and kyanite where the pressure was high, and andalusite where the pressure was low. 3. High-grade amphibolite facies, politic rocks containing 50.

(64) sillimanite. 4. Hornblende-granulite facies. The gray line in Ti content histogram presumably indicates the maximum possible Ti in hornblende of respective metamorphic facies. Values plotting above this gray line may come from hornblende which have not been completely separated from inclusions of sphene, or my error in analysis. The variation of compositions below the gray line may be attributed to a deficiency of bulk rock composition in Ti, to the coexistence of different Ti-containing minerals with hornblende, and to the rough limitation of the metamorphic facies. There is a clear trend of increasing Ti with metamorphic grade, maximum possible Ti of hornblendes should be essentially a function of metamorphic temperature. The Al. Ⅵ. and Si content of hornblendes show clearly the pressure of. hornblendes crystalized environment. The Al. Ⅵ. and Si content of. hornblendes from some regional metamorphic terrains which are petrographically well-known is plotted in Fig. 18b~25b. It can be seen that hornblendes from low-pressure regional metamorphic facies series from sample 10CC02A to 10CC02E, are relatively low in Al 51. Ⅵ. and Si,.

(65) and plot well below the line of 5 kbar (Raase, 1974). Hornblende plotting above the line in the Al. Ⅵ. and Si content diagram should be formed at. pressure above approx. 5 kbar, and hornblendes below this line probably are formed lower pressure. Some hornblendes plotting below the gray line may be formed at high pressure in rocks of exceptional composition. The Al. Ⅵ. and Si content diagram permits a rather good distinction. between hornblendes of low-pressure type and of high-pressure type of regional metamorphism. According to the nomenclature of amphiboles from Hawthorne et al. (1997), all of the amphiboles from TBSZ belongs to calcic amphiboles. The detailed classification is shown in Figure 17c and d to 24c and d. The number of subdivisions used in IMA 78 has been more than halved; silicic edenite and compound names like tschermakitic hornblende have been abolished, and the boundaries of the group have been revised. 5.1. 10CC02A In histogram of Ti content, hornblendes in rocks of 10CC02A (Fig. 17a) appear at four different metamorphic facies. Amount of hornblendes with middle to low content of Ti, belong low-grade amphibolite facies 52.

(66) and greenschist facies, following is hornblende granulite facies and high-grade amphibolite facies. From thin section of Plane-polarized light (PPL) (Fig. 17c), it is obviously observed that the rim of hornblende with blue color, indicating the high Ti content, so we compared different structure zones of hornblende with EPMA (Fig. 17d). The rim structure zone (red) of hornblendes appear at low-grade amphibolite facies and greenschist facies, indicating a low temperature event occurred. Most of the core structure zone (dark red) of hornblendes plot on low-grade amphibolite facies, and hornblendes evolved biotite at granulite facies, suggesting the relative high temperature. In Fig. 17b, both the rim (triangle) and core (circle) of hornblendes are under the pressure of 5 kbar. Another group of biotitic hornblende (square) formed at pressures probably above 5 kbar. 10CC02A in the Sm domain, we can infer that biotitic hornblende crystallized at relative high pressure environment, consistent with the high temperature of high Ti content in granulite facies. Other hornblendes are below 5 kbar, occurred at low-grade amphibolite facies. 10CC02A amphiboles plot on magnesiohornblende, tschermakite, 53.

(67) pargasite, and edenite region (Fig 17e, f), biotitic hornblende and most of the rim of hornblendes belong to magnesiohornblende, and the core of hornblendes belong to pargaslite and edenite.. 54.

(68) (a). (b b). (c). (d). (f). (e) D Diagram parametters: CaB ≥ 1.50; (Na ( + K)A < 0.50;; CaA < 0.50. Diagram D paramete ers: CaB ≥ 1.50; (N Na + K)A ≥ 0.50; Ti < 0.50. Figg. 17. The metamorp phic faciess of hornb blendes and d amphiboole classsificationn of samplle 10CC022A. (a) Histogram of o Ti conteent of horrnblendes in rocks of o four meetamorphicc facies. (R Raase, 19774) (b) Ⅵ. ure region Rellation betw ween Al and Si off hornblen ndes from low-pressu l nal mettamorphicc terrains and a from hhigh-presssure metam morphic teerrains 55.

(69) (Raase, 1974). (c) PPL thin section of 10CC02A (d) SEM image of 10CC02A (e) and (f) Classification of the calcic amphiboles (Hawthorne et al., 1997).. 56.

(70) 5.2. 10CC02B In histogram of Ti content, hornblendes in rocks of 10CC02B (Fig. 18a) appear at three different metamorphic facies. Amount of hornblendes with middle to low content of Ti, belong low-grade amphibolite facies and greenschist facies, few in high-grade amphibolite facies. Most of the rim structure zone (orange) and the core structure zone (brown) of hornblendes appear at low-grade amphibolite facies and greenschist facies. Furthermore, in PPL thin section (Fig. 18c), there were a block of hornblende show blue color, indicating a high temperature event occurred. In Fig. 18b, both the rim (triangle) and core (circle) of hornblendes are under the pressure of 5 kbar, but higher than 10CC02A in average, it is reasonable that 10CC02B located at cataclacite domain. 10CC02B amphiboles plot on magnesiohornblende, tschermakite, pargasite region (Fig. 18e, f), most of the core of hornblendes belong to magnesiohornblende, and the rim of hornblendes belong to pargaslite and magnesiohornblende.. 57.

(71) (a). (b b). (d d). (cc). (e). (f). Diaagram parameterss: CaB ≥ 1.50; (Naa + K)A < 0.50; C CaA < 0.50. gram parameters: CaB ≥ 1.50; (Na + K)A ≥ 0.50; Ti < 0.50 Diag. Figg. 18. The metamorp phic faciess of hornb blendes and d amphiboole classsificationn of samplle 10CC022B. (a) Histogram of o Ti conteent of horrnblendes in rocks of o four meetamorphicc facies. (R Raase, 19774) (b) Ⅵ. ure region Rellation betw ween Al and Si off hornblen ndes from low-pressu l nal mettamorphicc terrains and a from hhigh-presssure metam morphic teerrains 58.

(72) (Raase, 1974). (c) PPL thin section of 10CC02B (d) SEM image of 10CC02B (e) and (f) Classification of the calcic amphiboles (Hawthorne et al., 1997).. 59.

(73) 5.3. 10CC02C In histogram of Ti content, hornblendes in rocks of 10CC02C (Fig. 19a) appear at two different metamorphic facies. Amount of hornblendes with high content of Ti, belong high-grade amphibolite facies, few with low contentof Ti in low-grade amphibolite facies. Most of the rim structure zone (yellow) and the core structure zone (green) of hornblendes appear at high-grade amphibolite facie, only one core at low-grade amphibolite facies; furthermore, in PPL thin section (Fig. 19c), hornblende show blue color at edge. In Fig. 19b, both the rim (triangle) and core (circle) of hornblendes are under the pressure of 5 kbar, and higher than 10CC02B, and the relative high pressure consistent with high Ti of high-grade amphibolite facies. 10CC02C amphiboles plot on magnesiohornblende, tschermakite, pargasite region (Fig. 19e, f), most of the core and the rim of hornblendes belong to magnesiohornblende, and tschermakite.. 60.

(74) (a). (b b). (d d). (c). (f). (e) D Diagram parameteers: CaB ≥ 1.50; (N Na + K)A < 0.50; CaA < 0.50. Na + K)A ≥ 0.50; Ti T < 0.50 Diiagram parameterrs: CaB ≥ 1.50; (Na. Figg. 19. The metamorp phic faciess of hornb blendes and d amphiboole classsificationn of samplle 10CC022C. (a) Histogram of o Ti conteent of horrnblendes in rocks of o four meetamorphicc facies. (R Raase, 19774) (b) Ⅵ. ure region Rellation betw ween Al and Si off hornblen ndes from low-pressu l nal mettamorphicc terrains and a from hhigh-presssure metam morphic teerrains 61.

(75) (Raase, 1974). (c) PPL thin section of 10CC02C (d) SEM image of 10CC02C (e) and (f) Classification of the calcic amphiboles (Hawthorne et al., 1997).. 62.

(76) 5.4. 10CC02D In histogram of Ti content, hornblendes in rocks of 10CC02D (Fig. 20a) appear at only one metamorphic facies, which is high-grade amphibolite facies. All the core structure zone of hornblendes appear at high-grade amphibolite facie, and in PPL thin section (Fig. 20c), hornblende show blue color at edge. In Fig. 20b, all core of hornblendes are under the pressure of 5 kbar, but it is the highest in 10CC02, consistent with the growth of garnet and Sm domain, suggesting the high-grade amphibolite facies. All of the core of 10CC02D amphiboles plot on tschermakite region (Fig. 20e).. 63.

(77) (a). (b) (. (c). (d d). (e) Diagram param meters: CaB ≥ 1.50 0; (Na + K)A < 0.550; CaA < 0.50. Figg. 20. The metamorp phic faciess of hornb blendes and d amphiboole classsificationn of samplle 10CC022D. (a) Histogram of o Ti conteent of horrnblendes in rocks of o four meetamorphicc facies. (R Raase, 19774) (b) Ⅵ. ure region Rellation betw ween Al and Si off hornblen ndes from low-pressu l nal 64.

(78) metamorphic terrains and from high-pressure metamorphic terrains (Raase, 1974). (c) PPL thin section of 10CC02D (d) SEM image of 10CC02D (e) Classification of the calcic amphiboles (Hawthorne et al., 1997).. 65.

(79) 5.5. 10CC02E In histogram of Ti content, hornblendes in rocks of 10CC02E (Fig. 21a), the two spot of core structure zone of hornblendes appear at high-grade amphibolite facies, and in PPL thin section (Fig. 21c), biotite replaced most hornblende. In Fig. 21b, the two core of hornblendes are under the pressure of 5 kbar, with low Al content in deformed vein of C domain. The two core of 10CC02E amphiboles plot on Pargasite region (Fig. 21e).. 66.

(80) (a). (b b). (c). (d d). (e) Diagram param meters: CaB ≥ 1.50; (Na + K)A ≥ 0.550; Ti < 0.50. Figg. 21. The metamorp phic faciess of hornb blendes and d amphiboole classsificationn of samplle 10CC022E. (a) Hisstogram of Ti conteent of horrnblendes in rocks of o four meetamorphicc facies. (R Raase, 19774) (b) Ⅵ. ure region Rellation betw ween Al and Si off hornblen ndes from low-pressu l nal 67.

(81) metamorphic terrains and from high-pressure metamorphic terrains (Raase, 1974). (c) PPL thin section of 10CC02E (d) SEM image of 10CC02E (e) Classification of the calcic amphiboles (Hawthorne et al., 1997).. 68.

(82) 5.6. 10CC03A In histogram of Ti content, hornblendes in rocks of 10CC03A (Fig. 22a) appear at three different metamorphic facies. Amount of hornblendes with middle to low content of Ti, belong low-grade amphibolite facies and, only two samples is high-grade amphibolite facies. From Plane-polarized light (PPL) thin section (Fig. 22c), it is obviously observed that hornblende were alternated by biotite. Both of the core (blue) and rim (cyan) structure zone of hornblendes plot from greenschist facies to high-grade amphibolite facies. In Fig. 22b, both the rim (triangle) and core (circle) of hornblendes are under the pressure of 5 kbar in a wide range. 10CC03A amphiboles plot on magnesiohornblende, and pargasite region (Fig 22e, f), and most of the core of hornblendes belong to magnesiohornblende, and tschermakite.. 69.

(83) (a). (b b). (d)). (c). (e). (f). Diaagram parameterrs: CaB ≥ 1.50; (Naa + K)A < 0.50; C CaA < 0.50. Diagram parameters: CaB ≥ 1.50; (N Na + K)A ≥ 0.50; Ti T < 0.50. Figg. 22. The metamorp phic faciess of hornb blendes and d amphiboole classsificationn of samplle 10CC033A. (a) Histogram of o Ti conteent of horrnblendes in rocks of o four meetamorphicc facies. (R Raase, 19774) (b) Ⅵ. ure region Rellation betw ween Al and Si off hornblen ndes from low-pressu l nal mettamorphicc terrains and a from hhigh-presssure metam morphic teerrains 70.

(84) (Raase, 1974). (c) PPL thin section of 10CC03A (d) SEM image of 10CC03A (e) and (f) Classification of the calcic amphiboles (Hawthorne et al., 1997).. 71.

(85) 5.7. 10CC03B In histogram of Ti content, hornblendes in rocks of 10CC03B (Fig. 23a) appear at two metamorphic facies, both of rim (purple) and core (dark purple) belong low-grade amphibolite facies and greenschist facies. From Plane-polarized light (PPL) thin section (Fig. 23c), it is obviously observed some hornblende were changed to blue color. In Fig. 23b, both the rim (triangle) and core (circle) of hornblendes are under the pressure of 5 kbar in a wide range. 10CC03B amphiboles plot on magnesiohornblende, and pargasite region (Fig 23e, f), and most of them belong to magnesiohornblende, and tschermakite.. 72.

(86) (a). (b). (c). (d)). (e). (f). Diiagram parameterrs: CaB ≥ 1.50; (N Na + K)A < 0.50; C CaA < 0.50. Diaagram parameters: CaB ≥ 1.50; (Na + K)A ≥ 0.50; Ti < 0.50. Figg. 23. The metamorp phic faciess of hornb blendes and d amphiboole classsificationn of samplle 10CC033B. (a) Histogram of o Ti conteent of horrnblendes in rocks of o four meetamorphicc facies. (R Raase, 19774) (b) Ⅵ. ure region Rellation betw ween Al and Si off hornblen ndes from low-pressu l nal mettamorphicc terrains and a from hhigh-presssure metam morphic teerrains 73.

(87) (Raase, 1974). (c) PPL thin section of 10CC03B (d) SEM image of 10CC03B (e) and (f) Classification of the calcic amphiboles (Hawthorne et al., 1997).. 74.

(88) 5.8. 10CC03C In histogram of Ti content, hornblendes in rocks of 10CC03C (Fig. 24a) appear at two different metamorphic facies, both of rim (purple) and core (dark purple) belong low-grade amphibolite facies and greenschist facies. From Plane-polarized light (PPL) thin section (Fig. 24c), it is obviously observed the rim of hornblende were changed to blue color. In Fig. 24b, both the rim (triangle) and core (circle) of hornblendes are under the pressure of 5 kbar, in a wide range, the same as 10CC03B. 10CC03C amphiboles plot on magnesiohornblende, and pargasite region (Fig 24e, f), and most of them belong to magnesiohornblende, and tschermakite.. 75.

(89) (a). (b b). (d d). (cc). (e). (f). Diagram parameters: CaB ≥ 1.50; (N Na + K)A < 0.50; C CaA < 0.50. Na + K)A ≥ 0.50; Ti T < 0.50 Diiagram parameterrs: CaB ≥ 1.50; (N. Figg. 24. The metamorp phic faciess of hornb blendes and d amphiboole classsificationn of samplle 10CC033C. (a) Histogram of o Ti conteent of horrnblendes in rocks of o four meetamorphicc facies. (R Raase, 19774) (b) Ⅵ. ure region Rellation betw ween Al and Si off hornblen ndes from low-pressu l nal 76.

(90) metamorphic terrains and from high-pressure metamorphic terrains (Raase, 1974). (c) PPL thin section of 10CC03C (d) SEM image of 10CC03C (e) and (f) Classification of the calcic amphiboles (Hawthorne et al., 1997).. 77.

(91) V. Interpretation 1. Geochronology Base on the U-Pb results above, magmatic event occurred during 618 Ma. Followed by high grade metamorphic event during 610-596 Ma. However, the tectonic significance of 2 Ga core remain unclear, so we need to decipher the nature of protolith is granite or sandstone (Fig. 7-12). In order to confirm they were metasedimentry rocks, the zircon Hf isotope analysis is necessary. In sample 10CC02 and 10CC03, εHf(T) values was depleted with time, the positive value in the cores age about 2 Ga indicating mantle source, and negative in the rims age ~600 Ma with ancient crust material. Sample 10CC04 divided into two groups depending on εHf(T) values, the positive one is the core of zircons, negative is rim, the mixing Hf indicating there were not only magmatic but also metamorphic event happened, that is an arc environment. The negative εHf(T) value on 10CC05 observed exhibit a decrease trend, suggesting the continental crust material (Fig. 16). From the result of the K-feldspar 40Ar/39Ar dating, step-heating patterns suggests that higher temperature is about 584.3~594.2 Ma, and lower temperature is 513.9~525.3 Ma. And the SEM image showed boundinage of K-feldspar, 78.

(92) and CL image shows albitization of K-feldspar, so K-feldspar is crystallized before albite, during former metamorphic event (Fig. 15). 2. Geochemistry and petrography In whole rock major elements composition analyses, our samples belong to gabbroic diorite to diorite with metaluminous feature indicating meta-igneous rock type, but in the calc-alkaline field, consistent with continental crust (Fig. 13). In trace elements analyses, all the rare earth elements content are much low than Neoproterozoic granitoids, and most of the heavy rare earth element under the detection limit, nearly depleted, light rare earth element left, with low content (Table 4, Fig. 14), suggesting that TBSZ rocks were metasedimentary rocks. From hornblende analysis result, it is obviously that part of hornblende was replaced by biotite from thin sections of PPL, consistent with the high content of Ti in high temperature and the high Al content in high pressure environment of high-grade amphibolite to granulite facies (Fig. 17), so hornblendes recorded the retrograde metamorphism. Otherwise, we proceeded EPMA to analyze the geochemistry composition of minerals (Table 8) deciphering the change of 79.

(93) metamorphic facies precisely. In ACF diagram, minerals of TBSZ sample plot in amphibolite and greenschist facies field (Fig. 25). The normative compositions were calculated assuming an Fe2O3/FeO ratio of 0.3 (Middlemost, 1989). In sample 10CC02A-E, the mineral composition of Pl + Hbl ± Bt ± Ep belongs to amphibolite facies (Fig. 25a); otherwise, the mineral composition of sample 10CC03A-C is Chl + Ab + Ep + Act  Qtz, which belongs to greenschist facies (Fig. 25c) (Winter, 2001). The most characteristic mineral assemblage of the two groups samples is Pl + Hbl → Ep + Chl, suggesting the mineral replacement change from amphibolite facies to greenschist facies during retrograde metamorphism. In SEM image of 10CC02B (Fig.25b), it is obviously that magnesionhornblende (Mg-Hbl) was replaced by biotite. A continuous decrease of the metamorphic temperature is indicated by the gradual replacement of chlorite by biotite (Fig.25d). The two metamorphic events accompanied with replacement, in amphibolite facies, hornblendes were replaced by biotites, and this is the reaction: Hbl + (Mg, Fe)2+ + K+ + SiO2 + H2O → Bt + Ca2+ + 40H+. The following metamorphism is under greenschist facies, and chlorites replaced biotites: Bt → Chl + K+ + Ti+ SiO2.. 80.

(94) (a). (b). (c)). (d). Figg. 25. The ACF A diag gram and S SEM imag ge. (a) Thee mineral aassemblag ge of ssample 100CC02B plot on AC CF diagram m. (b) The SEM imaage of 10C CC02B, shhowing the replacem ment of Mg-Hbl M by Bt. (c) Thhe minerall assemblage of o sample 10CC03A A plot on ACF A diagrram. (d) T The SEM imaage of 10C CC03A, sh howing th e replacem ment of Ch hl by Bt.. 81.

(95) VI. Discussion 1. Reconstruction tectonic evolution Reconstructions of ancient supercontinents have been a topic of wide interest in Earth Sciences over many years. According to zircon U-Pb dating, Hf isotope results above, the core age group of ~2 Ga not only recorded the magmatism event of Eburnean orogeny indicating the collision of South America and West Africa cratons in Paleoproterozoic, but also the formation of accretionary orogens. The result provide the evidence of South America and West Africa cratons were amalgamated at 2.1-2.0 Ga formed Atlantica, which was parts of the Paleo-Mesoproterozoic Columbia supercontinent (Zhao et al., 2002a). The Tcholliré-Banyo fault, occurred primarily during the Paleoproterozoic and involved both crustal recycling and crustal accretion processes. The Paleoproterozoic crust was extensively reworked during the Neoproterozoic (Pan African orogeny). 1.1. Possible protolith Eburnean sedimentary From whole rock major and trace elements composition analyses, the calc-alkalic, metaluminous characteristic of gabbroic diorite to diorite 82.

(96) (Table 3 and 4, Fig. 13 and 14) are consistent with post-collisional environment (England and Thompson, 1986). Calc-alkaline batholiths typically occur in post-collisional settings during large relative movements of terranes along major shear zones (Barbarin, 1999). Trace elements composition reflects the metasedimentary characteristics of low content in LREEs and depleted in HREEs. These metasedimentary rocks might consistent with Yaoundé belt and Lom basins Mesoproterozoic Sm–Nd model ages of 1600–1000 Ma occur in northern Cameroon, northwest of the Tcholliré-Banyo fault. Toteu et al. (2001) remarked that Yaoundé belt and Lom basins have substantial amounts of detritus from older, mainly Paleoproterozoic, crustal material. 2. Syn-Pan-African metamorphism and magmatism The Pan-African was interpreted as a tectonic-thermal event, during ~870 to ~550 Ma, surrounding older cratons. The term Pan-African is now used to describe tectonic, magmatic, and metamorphic activity of Neoproterozoic to earliest Palaeozoic age, especially for crust that was once part of Gondwana (Kröner and Stern, 2004). With the dominant Neoproterozoic age indicated by our U-Pb dating, Hf isotope ratios and 83.

(97) 40. Ar/39Ar dating, that corresponds to Pan-African orogeny. In zircon. 10CC04 of granite sample, which is separated in two group depending on εHf(T) values (Fig. 16), the positive one is formed from magma derived material; the negative one is caused from the amalgamation between ancient cratons of Gondwana. The change of metamorphic facies from amphibolite to greenschist facies recorded by petrological analysis, pointing out that the metamorphism occurred at the activities of TBSZ and CASZ. Microstructural analysis and the electron probe micro analysis data showed that the mineral assemblage was Pl + Hbl ± Bt ± Ep at the first deformation period. From mineral contact relationship, it can be inferred hornblende was replaced by biotite. According to EPMA results, amphiboles growth crossing granulite, amphibolites and greenschist facies (Fig. 17-24). Moreover, some of hornblendes had high Ti and Al. Ⅵ. content, which were considered the residual replacing by biotite in high-grade amphibolites facies. The mineral assemblage of the second deformation event was Chl + Ab + Ep + Act  Qtz, some biotite was replaced by chlorite, indicated that the temperature and pressure conditions was lower greenschist facies. Based on our geochronology and geochemistry data, we rebuilt geothermal chronology evolution of TBSZ 84.

(98) (Fig. 26). Since the Eburnean orogeny, South America collided with West Africa block, the igneous events happened resulting magmatic protolith, and zircons recorded core ages about 2000 Ma (Rogers and Santosh, 2003). After that, the rocks were uplifted onto the surface, consistent with a period of Neoproterozoic crustal rifting and basin formation of Yaoundé belt and Lom basins. To continue, the amalgamation of Gondwana made magmatic event accompany with metamorphism recorded the Pan-African activity at about 600 Ma. The collision of Congo craton and Sahara block recorded a ductile structural event happened with these rocks recrystallized under amphibolite facies. Otherwise, from the K-feldspar of 40Ar/39Ar dating, the step heating patterns of higher temperature is 584.3~594.2 Ma, lower temperature is 513.9~525.3 Ma, and the SEM image showed boundinage of K-feldspar, which is albitized, so K-feldspar is crystallized before this metamorphic event, and replaced by albite during sinistral shearing activity of TBSZ (Wu, 2012) under greenschist facies (Fig. 26).. 85.

(99) (aa). (b). Maagmatic Ev vent. Figg. 26. (a) Relative R prrobability diagram of o zircon U–Pb U agess of the co ore of zzircons. (bb)The geotthermal chhronology y evolution n of TBSZ Z and CAS SZ. 86.

(100) VII. Conclusions 1. The wide spread age range of 2000 ± 200 Ma and extremely low concentration of trace elements revealed the crystalline rocks we analyzed within the TBSZ and CASZ are composed of metasedimentary rocks with sediment source of 2 Ga magmatic rocks from the Eburnean orogeny. 2. The 696 ~ 514 Ma metamorphic zircon rim and petrology analysis showed these metasedimentary rocks experienced peak metamorphism under amphibolite facies post the 600 Ma Pan-African magmatic event. 3. The 600 Ma Concordia zircon ages and wide range of εHf (T) values from +1.0 to +10.5 and −11.2 to −18.5 of granite sample is determined as the crystallization age of the Pan-African magmatism under arc setting as the Eastern Sahara Block amalgamate with the Congo and West African Craton. 4. The metasedimentary rocks later went through lower greenschist facies retrograde metamorphism during 589 ~ 519 Ma indicated by the 40. Ar/39Ar age of K-feldspar accompanying the structural activities of. TBSZ.. 87.

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