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Post-collisional, potassic monzonite–minette complex (Shahewan) in the Qinling Mountains (central China): 40Ar/39Ar thermochronology, petrogenesis, and implications for the dynamic setting of the Qinling orogen

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Post-collisional, potassic monzonite–minette complex (Shahewan)

in the Qinling Mountains (central China):

40

Ar/

39

Ar

thermochronology, petrogenesis, and implications

for the dynamic setting of the Qinling orogen

Fei Wang

a,*

, Xin-Xiang Lu

b

, Ching-Hua Lo

c

, Fu-Yuan Wu

a

, Huai-Yu He

a

,

Lie-Kun Yang

a

, Ri-Xiang Zhu

a

aPaleomagnetism and Geochronology Laboratory of State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

bHenan Institute of Geology, Zhengzhou 450053, China cDepartment of Geology, National Taiwan University, Taipei, Taiwan Received 30 October 2006; received in revised form 5 June 2007; accepted 22 June 2007

Abstract

The nondeformed, postcollisional Shahewan monzonite–minette complex in the North Qinling orogenic belt, central China, is composed of potassic monzonites and mafic minette dykes. The monzonites and minette dykes were coevally intruded as evidenced by a zircon U–Pb age of 211 ± 2 Ma for the monzonites and a40Ar/39Ar age of 209.0 ± 1.4 Ma on the biotite from the minette dykes.

These dates also indicate that the Shahewan complex postdated shortly the cessation of convergent deformation in the Qinling orogenic belt. The K-feldspar 40Ar/39Ar MDD modeling result combined with the zircon U–Pb age reveals a rapid average cooling history

(11.5°C/Ma) for the complex from 211 to 150 Ma, implying a fast exhumation within a short period of time.

The minettes take features of ultrapotassic rocks. Their geochemical characteristics, such as high MgO (Mg# up to 80), low TiO2

(0.38–0.62 wt.%) and FeO (2.52–3.42 wt.%), enrichment in LILEs and LREEs, and depletion in HFSE, together with primitive initial

87

Sr/86Sr ratios (0.70444–0.70562) and slightly negative eNd(t) ( 3.4 to 2.5), suggest that they were derived from an enriched refractory

lithospheric mantle. The monzonites have the similar geochemical (incompatible element enrichment, Nb, Ta, Ti depletion) and isotopic (initial87Sr/86Sr: 0.70513–0.70646; eNd(t): 0.5 to 3.6) imprints, indicating they were derived from a common source.

The dynamic setting in which the Shahewan complex formed is discussed. The Shahewan complex formed most likely in a postcol-lisional extensional setting. Its formation requires a high enough temperature to partially melt an enriched, refractory mantle source. Considerable uplift is necessitated for the rapid exhumation. These may be explained by convective mantle thinning or slab-breakoff mechanisms. The enrichment features in the lithospheric mantle source can be interpreted by injection of small percentage (<1%) partial melts from the convective mantle, or by addition of the metamorphic fluids from the subducted South China lithosphere.

Ó 2007 Elsevier Ltd. All rights reserved.

Keywords: The Qinling orogen; Potassic monzonite–minette complex; Petrogenesis;40Ar/39Ar thermochronology; Dynamic setting

1. Introduction

The Qinling–Dabie orogenic belt, which separates the eastern Asian continent into the North and South China

Blocks, is crucial to understanding the evolution of the Eastern Asian dynamics. Magmatic rocks in the Qinling belt provide a record of the thermal and geochemical evo-lution of the deep lithospheric root in this developing oro-gen. The evolving tectonic regimes of orogenic belts are typically marked by changes in the composition of associ-ated magmatism, therefore granitic intrusions in the 1367-9120/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jseaes.2007.06.002 *

Corresponding author. Tel.: +86 10 62007357; fax: +86 10 62010846. E-mail address:wangfei@mail.iggcas.ac.cn(F. Wang).

www.elsevier.com/locate/jaes Journal of Asian Earth Sciences 31 (2007) 153–166

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Qinling belt may provide a tool to pry into the dynamic set-ting at the deep. Postcollisional, potassic magmatism is a common feature in many collisional orogenic belts around the world, although its origin is poorly understood (Turner et al., 1996). Clearly, the potassic rocks that postdated col-lision offer a potentially important window into the deep compositional structure of the Qinling orogen.

Various earlier studies have been carried out on the gra-nitic rocks in the Qinling orogenic belt (e.g. Zhang et al., 1996; Lu et al., 1999; Sun et al., 2002a; Wang et al., 2005). However these studies remain controversial about the petro-genesis of these rocks and the dynamic context. The mecha-nisms proposed for the generation of these rocks are diversified and may be classified into three groups. Firstly, these rocks may have originated by partial melting of the crustal rocks but mixed with the primitive magma derived from the mantle (Wang et al., 2005). Secondly, the potassic rocks may be the products of the partial melting of the sub-ducted crust of the South Qinling (Zhang et al., 1996). And finally, these rocks are derived from an enriched lithospheric mantle source beneath the North Qinling, triggered by slab-breakoff of the subducted South China plate (Sun et al.,

2002a) or mantle delamination (Lu et al., 1999). These differ-ent petrogenetic models reflect the diverse perspectives into the dynamic setting in which these rocks are generated. For example, some authors refer to these rocks as ‘‘syn-col-lisional’’ (Zhang et al., 1996), some name them ‘‘syn-oro-genic’’ or ‘‘post-collisional ’’ rocks (Sun et al., 2002a), and some attribute them to ‘‘post-orogenic’’ (Lu et al., 1999).

In this paper, based on experimental data, the Shahewan complex is first described according to age, location, geo-chemical and isotopic composition, and cooling history, then the constraints on the sources of monzonites and minettes of the complex are discussed, respectively. Finally, these observations are employed to assess the magmatism in the context of geodynamic models for the Qinling oro-gen during the Late Triassic time.

2. Geological setting and the Shahewan monzonite–minette complex

The Qinling orogenic belt is regarded as the western part of the Qinling–Dabie ultrahigh-pressure metamor-phic belt (Fig. 1) (Zhang et al., 1995; Li and Sun,

Fig. 1. Geological sketch of the Qinling orogenic belt showing the locations of the granitoids and the Shahewan complex (compiled afterZhang, 1994; and

Lu et al., 1999; and from new field observations). The insets show the major plates in China and the studied area, and the detail of the Shahewan complex and the sample sites.

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1996; Meng and Zhang, 1999), and an important tectonic domain in eastern Asia (Mattauer et al., 1985; Yang et al., 1986; Zhang et al., 1989, 1995; Xu et al., 2000;

Meng and Zhang, 2000). The present studies propose

that the studied area of this belt could be divided into the South and North Qinling zones, and the South and North China Blocks (Fig. 1). Two sutures have been identified, i.e. the Shangdan suture between the South and North Qinling zones, and the Mianlue suture between the South Qinling zone and the South China Block (Zhang et al., 1989, 1995; Zhang, 1994; Meng and Zhang, 1999) (Fig. 1).

The tectonic evolution history of the Qinling orogenic belt is seriously disputed. Some geologists suggest two stages of development: Caledonian oceanic crust conver-gence and, Indosinian detachment in the upper crust (Mattauer et al., 1985; Xu et al., 2000; Zhang et al., 1989, 1995; Ratschbacher et al., 2003). Others argue that the Qinling orogenic belt was built up during the Indosian orogeny by the collision between the North and South China blocks (Sengor, 1985; Hsu¨ et al., 1987; Reischmann et al., 1990; Kro¨ner et al., 1993; Li and Sun, 1996; Sun et al., 2002a,b).

In the Qinling orogenic belt, granitoids are widely dis-tributed, especially in the north Qinling zone (Fig. 1). These granitoids were intruded mainly at three separate stages. The granites at the first stage with ages of 444– 357 Ma (U–Pb, Lu, 2000; Sun et al., 2002b), whose dis-tribution is constrained to the North Qinling zone, are characterized by intensive deformation. The granites at the second stage, intruded during 220–205 Ma (U–Pb, Lu, 2000; Sun et al., 2002a) and scattered in both the South and North Qinling zones, are nondeformed. The third stage granites are contained in the North Qinling zone and the North China Block, and were formed dur-ing the late Mesozoic time.

The Shahewan monzonite–minette complex occurs on the Shangdan suture with an outcrop area of 120 km2 (Fig. 1). Its country rocks include the gneiss of the high-grade metamorphosed Qinling Group, low-grade metamorphosed volcanic rocks of the Paleaozoic Danf-eng Group (487–397 Ma, Reischmann et al., 1990; Xiao et al., 1988), and the sandy slate of the Devonian Liul-ing Group. The contacts between the complex and coun-try rocks are approximately vertical. Xenoliths of the older country rocks are commonly seen in the marginal phase.

The Shahewan complex comprised petrologically mas-sive nondeformed medium- to coarse-grained hornblende-biotite monzonites with alkali feldspar megacrysts, most of which are mantled by plagioclase (rapakivi texture,Lu et al., 1999). The monzonites should be classified as I-type due to bearing hornblendes; mafic dykes and enclaves of minettes can be seen widely. Usually, the dykes range from a few centimetres to several metres in width and mingled with the monzonites, forming swarms of enclaves (Lu et al., 1999).

3. Analytical techniques

3.1. Major and trace elements, and Sr, Nd isotopes

Major element’s abundance was determined by using X-ray fluorescence spectrometry (XRF) method with a Philips PW 1400 spectrometer at IGGCAS (Institute of Geology and Geophysics, Chinese Academy of Sciences). Analytical uncertainties range from 2% to 5%. Trace elements including REE elements were analyzed using ICP-MS method also at IGGCAS, with analytical uncertainties less than 5%.

Sr, Nd isotopic analyses were conducted at Chenggong University, Taiwan. The concentrations of Rb, Sr, Sm and Nd were obtained by using isotope dilution method; a mixed 87Rb–84Sr–149Sm–150Nd spike solution was used. Isotopic ratio measurements were made on a multicollector MAT-262 thermal ionization mass spectrometer. Measured Sr isotope ratios were normalized to86Sr/88Sr = 0.1194; the

87

Sr/86Sr of the Sr standard NBS-607 was 1.200393 ± 12 (2r, n = 6). Measured Nd ratios were normalized to

144

Nd/146Nd = 0.7219; the average value of the

143

Nd/144Nd ratios of La Jolla standard is 0.511863 ± 7 (2r, n = 6). Analytical uncertainties are estimated to be <0.5–1.0% for both 87Rb/86Sr and 147Sm/144Nd ratios. Blanks during the course of this study averaged 0.4 ng for Rb, 0.2 ng for Sr, 0.06 ng for Sm and 0.05 ng for Nd. For plotting on a diagram, the Sr and Nd isotope ratios were age corrected to 211 Ma.

3.2. Zircon U–Pb analyses

One sample (QM-2) from the monzonites was chosen for zircon U–Pb dating. The analytical work was per-formed at the Laboratory of Isotope Geology, Tianjin Institute of Geology and Mineral Resources.

The zircon fractions were separated from the crushed and ground rocks using a continuous sieve, a Wifley table, heavy liquids and a Frantz isodynamic magnetic separator. The zir-cons were removed stepwise and acting non-magnetically at a current of 1.7 A, and 2.5° vertical and 5° horizontal tilt. The concentrates were further split into size-fractions from which the most transparent, prismatic and inclusion-free crystals were hand-picked. The analyzed zircons were usually trans-parent and pale-rosy colour with a length-width ratio of 2.5:1, and well developed euhedral prisms and simple pyra-midal endings. Four fractions of zircons were analyzed according to their differences in color and morphology.

The chemical procedure, including the decomposition of the crystal and the extraction of uranium and lead, was per-formed essentially according to the method described by Krogh (1973). A mixed208Pb–235U spike was added to the solution. Both uranium and lead were loaded on Re fila-ments using nitric acid and phosphoric acid/silica gel tech-niques respectively. Both uranium and lead isotopic ratios were measured on a VG354 mass spectrometer with a Daly collector. Mass-fractionation correction of 0.1% amu 1for Pb was made, based on measurements of NBS ARM 981.

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Common lead in the sample was corrected for using the growth curve ofStacey and Kramers (1975)with the follow-ing ratios: 206Pb/204Pb = 16.1, 207Pb/204Pb = 15.4 and

208

Pb/204Pb = 35.8. The calculation of elemental and isoto-pic compositions is processed using the program of PBDAT (Ludwig, 1991a) and the average mean206Pb/238U age is cal-culated using the ISOPLOT program (Ludwig, 1991b). The decay constants recommended by the IUGS Subcommission on Geochronology (Steiger and Ja¨ger, 1977) are applied. The total blank of lead and uranium are, on average, 0.03– 0.05 ng and 0.002–0.004 ng, respectively.

3.3.40Ar/39Ar analyses

40

Ar/39Ar step-heating experiments were carried out on a biotite from the minette dyke sample MT-1 at IGGCAS (Institute of Geology and Geophysics, Chinese Academy of Sciences), and on a K-feldspar from the monzonite sample QM-2 at National Taiwan University, Taiwan.

The minerals with grain sizes between 200 and 280 lm were obtained from the usual mineral separation tech-niques by using heavy liquids, Frantz magnetic separator, and finally handpicked under a binocular to remove all vis-ible impurities. Aliquots of mineral separates were wrapped in aluminum foil and stacked in quartz vial (at IGGCAS) or in an aluminum canister (at Taiwan National Univer-sity) along with the neutron flux monitor GA1550 biotite (at IGGCAS) or LP-6 biotite (at Taiwan National Univer-sity) which have K–Ar ages of 98.5 ± 0.8 Ma (Spell and McDougall, 2003) or 127.7 ± 1.4 Ma (Odin, 1982), respec-tively. The samples were irradiated at the H8 position in the 49-2 Reactor in Beijing or the VT-C position of the Tsing–Hua Open-Pool Reactor (THOR) at Tsing–Hua University, Taiwan. At both laboratories, samples were heated stepwise with a double vacuum resistance furnace. The released gas was purified with Zr–Al getters. The iso-topic composition was measured using a MM5400 mass spectrometer (at IGGCAS) or a Varian-MAT GDI50 mass spectrometer (at Taiwan National University). After cor-rections for mass discrimination, system blanks, radiomet-ric interference, 40Ar/39Ar ages were calculated according to40Ar*/39ArKratios and the J value obtained by analyses

of the monitors. Plateau ages were determined from three or more contiguous steps, comprising >50% of the 39Ar released, revealing concordant ages at the 95% confidence level (2r).

4. Results

4.1. Major and trace elements, and Sr–Nd isotopes

The chemical compositions of studied samples are shown in Table 1. The monzonite samples are relatively primitive with SiO2ranging from 64% to 67%. They have

high contents of alkalis, with K2O = 4.08–5.00% and

Na2O = 3.84–4.50%, but low abundances in total Fe

(Fe2O3+ FeO) (3.0–4.2%), MnO (0.05–0.20%), TiO2

(0.53–0.68%) and P2O5(0.25–0.32%). A12O3ranges from

14.10 to 15.28%. Variations of some selected oxides (FeO, Al2O3, TiO2, SiO2in wt.%) and trace elements (Ni,

V in ppm) with MgO (in wt.%) are shown inFig. 2. Chon-drite-normalized REE shows relatively smooth patterns without Eu anomalies (Fig. 3), the inclination to the right suggests enrichment of the light REE. In the spidergrams, the monzonites are enriched in incompatible trace elements such as Rb, Th, Ba (Fig. 3), and depleted in Nb, Ta and Ti. The minettes show much lower SiO2 contents (52.18–

57.86%), but K2O is as high as4.6%. The MgO contents

are in a range of 4.96–9.65, and calculated Mg# of70–80. In the primitive mantle normalized spidergrams (Fig. 3), the minettes also show enrichment in incompatible ele-ments with negative Sr anomalies. The REE patterns in Fig. 3indicate that the LREE contents are enriched against HREE with weak negative Eu anomalies (Fig. 3).

Whole-rock Sr–Nd isotopic data are listed in Table 1 and shown in Fig. 4. It manifests that the monzonites and minettes show quite similar Sr–Nd initial isotopic ratios (calculated at t = 211 Ma): monzonites in a range from 0.70513 to 0.70646 for 87Sr/86Sr and from 0.51218 to 0.51234 for 143Nd/144Nd, minettes in a range from 0.70444 to 0.70562 for 87Sr/86Sr and 0.51219 to 0.50224 for143Nd/144Nd, respectively. These ratios exhibit in a clus-ter close to the depleted mantle quadran (Fig. 4). Other granitoids of the Qinling belt (data from Zhang et al., 1991) are plotted inFig. 4either for comparison.

4.2. Geochronology

The zircon U–Pb analysis data are listed inTable 2and the concordia diagram is shown inFig. 5. The data from four zircon fractions of sample QM-2 are located on the concordia curve with a tight range of 206Pb/238U ratios, yielding a weighted mean age of 211±2 Ma, which is inter-preted as the time of emplacement.

40

Ar/39Ar dating on a biotite from the sample MT-1 was performed in order to constrain the emplacement time of minette dykes. The biotite displays a fairly flat age spec-trum with a well-defined plateau, giving an age of 209.4 ± 1.4 Ma (2r) over 95% of39Ark released (Table 3,

Fig. 6a). This age is quite consistent with the zircon U– Pb age of the host monzonite. Theoretically the40Ar/39Ar age just indicates the closure time of the Ar isotope system within the mineral during cooling and is later than its emplacement time, therefore the emplacement time of the minette dykes in the Shahewan complex should be rather close to the formation time of their host rocks. Therefore, we conclude that the minette dykes and their host monzo-nites of the Shahewan complex are coeval.

4.3. Thermochronology

In order to estimate the cooling rate after the emplace-ment of the monzonites, K-feldspar separates from sample QM-2 were analyzed for the Ar isotope distribution. The

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result displayed an age spectrum characterized by a gradi-ent of appargradi-ent ages increasing from 150 to 210 Ma (Fig. 6b). The abnormally old ages of the first few steps are possibly due to the excess Ar present in the margin of the mineral (Fig. 6b). Such increasing age pattern of K-feldspar may record its cooling history between 350– 150°C, and can be modeled by the multi-domain diffusion (MDD) theory (Lovera et al., 1989, 1991) although its

validity is questioned because of the complicated micro-structures within K-feldspar (Parsons et al., 1999; Reddy et al., 1999, 2001) and the potential influence from other diffusion mechanisms (Lee, 1995). A slab diffusion geome-try is thought to be justified because argon loss from K-feldspar during in-vacuo step-heating experiments can be best described by volume diffusion in a slab geometry (Lovera et al., 1997). After appropriate adjustment of the Table 1

Chemical compositions and Sr, Nd isotopes of the Shahewan pluton

Sample Monzonite Minette

QM-2 QM-3 QM-4 QM-5 QM-6 QM-7 MT-1 MT-2 MT-4 MT-5 Major elements (wt.%, by XRF) SiO2 67.36 67.00 65.38 64.18 64.20 64.00 54.90 55.50 57.86 52.18 TiO2 0.53 0.62 0.68 0.68 0.59 0.59 0.44 0.38 0.41 0.62 Al2O3 14.10 14.02 14.63 15.28 14.70 14.40 14.56 15.87 14.36 11.33 Fe2O3 1.78 1.00 1.33 1.18 1.02 1.05 1.12 1.23 1.04 1.40 FeO 2.17 2.38 2.37 2.52 2.41 2.57 3.42 3.16 2.52 3.30 MnO 0.05 0.11 0.12 0.11 0.19 0.20 0.18 0.14 0.15 0.05 MgO 2.05 2.04 2.15 2.16 2.20 2.20 5.42 4.96 5.55 9.65 CaO 2.63 2.62 2.94 3.08 5.50 3.00 5.52 5.51 4.85 5.87 Na2O 4.08 4.13 4.24 4.18 3.84 4.50 4.40 3.95 4.37 2.63 K2O 4.18 4.08 4.14 4.51 4.18 5.00 4.70 4.02 4.96 4.62 P2O5 0.25 0.30 0.29 0.27 0.25 0.32 0.31 0.30 0.24 0.42 LOI 0.58 0.48 0.80 1.10 0.70 1.25 3.67 3.02 2.56 7.20 Total 99.76 98.78 99.07 99.25 99.78 99.08 98.64 98.04 98.87 99.27 Mg# 50.7 54.5 53.5 53.6 56.0 54.6 70.3 69.1 75.5 80.3

Trace elements (in ppm, by ICP-MS)

Nb 8.00 12.30 13.80 11.30 15.21 14.96 16.71 14.80 15.28 17.64 Zr 193 240 190 180 163 196 120 130 155 161 Y 12.10 11.40 14.30 13.80 13.48 11.90 23.20 14.20 21.21 22.24 Sr 749 360 430 460 676 676 284 321 351 410 Rb 113 88 100 101 110 113 84 96 105 80 Co 10.84 11.32 9.29 8.87 9.22 10.35 20.22 22.40 29.54 20.91 V 60 66 55 48 61 53 100 115 127 109 Ta 1.24 0.84 1.14 0.83 0.95 1.14 1.10 0.74 1.22 0.55 Ni 40 42 37 33 49 42 52 48 46 46 Cr 158 56 57 68 100 85 95 112 88 123 Cu 6.80 7.23 6.21 6.34 5.72 7.43 5.81 6.35 6.51 7.24 Hf 7.90 6.40 6.40 4.60 5.90 4.54 6.72 4.77 5.40 Ba 1443 970 1080 1620 1149 1523 1127 1630 1737 1517 Ga 23.13 13.13 12.46 10.72 20.31 17.90 15.61 17.52 17.98 16.87 Th 14.27 11.20 15.85 15.34 16.50 13.22 11.54 15.87 14.18 13.42 U 4.17 3.62 5.25 5.14 4.47 3.75 3.34 4.55 5.12 3.84

Rare earth elements (in ppm, by ICP-MS)

La 38.60 49.40 50.40 42.30 34.77 35.05 58.30 52.90 87.04 66.27 Ce 70.5 74.6 94.7 77.4 66.7 62.9 115.2 87.6 134.3 113.5 Pr 7.85 10.60 11.70 10.40 7.48 6.75 10.89 9.21 11.73 11.33 Nd 28.1 27.5 32.9 30.2 26.1 22.4 42.3 33.7 49.2 43.5 Sm 5.22 5.68 6.94 6.26 4.42 3.89 8.28 6.75 7.98 7.16 Eu 1.08 1.29 1.39 1.42 1.49 1.55 1.71 1.06 1.14 1.43 Gd 3.85 3.67 4.32 4.20 4.04 3.74 4.48 4.31 3.96 4.27 Tb 0.60 0.57 0.67 0.63 0.52 0.65 0.58 0.54 0.66 0.62 Dy 3.12 2.51 3.28 3.14 2.58 2.40 2.38 1.96 2.31 2.04 Ho 0.58 0.54 0.69 0.66 0.49 0.44 0.48 0.45 0.66 0.57 Er 1.39 1.32 1.66 1.65 1.39 1.26 1.23 0.96 1.16 1.21 Tm 0.19 0.21 0.25 0.24 0.19 0.18 0.21 0.15 0.22 0.18 Yb 1.11 1.29 1.36 1.25 1.26 1.08 1.73 1.50 1.79 1.98 Lu 0.20 0.19 0.21 0.19 0.18 0.15 0.27 0.25 0.29 0.32 Sr, Nd isotope 87Sr/86Sr* 0.70574 0.70513 0.70548 0.70646 0.70612 0.70523 0.70447 0.70562 0.70444 0.70492 143Nd/144Nd* 0.51218 0.51224 0.51234 0.51223 0.51221 0.51230 0.51224 0.51224 0.51219 0.51221

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various model parameters, a model age spectrum can be obtained that closely matches the experimental results. The best-fit modeling parameters, such as the active energy

(E), size (q) and percentage of released gas (/) of K-feld-spar are summarized inTable 4. The modeling results are shown graphically inFig. 7.

1 2 3 4 Fe O 50 75 100 125 150 V 10 12 14 16 18 Al O 2 3 30 40 50 60 70 80 0 10 Ni MgO 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Ti O2 50 55 60 65 70 0 4 8 10 MgO Si O2 6 2 2 4 6 8

Fig. 2. Various oxide plots: ((a) FeO; (b) Al2O3; (c) TiO2; and (e) SiO2in wt.%) and trace element plots (d, V, and f, Ni, in ppm) vs. MgO (in ppm) for minettes and monzonites from the Shahewan complex. The open circle denotes minettes and the solid circle is for monzonites.

MT-1 MT-2 MT-4 MT-5 Monzonite QM-2 QM-3 QM-4 QM-5 QM-6 QM-7 1000 100 10 1000 100 10 QM-2 QM-3 QM-4 QM-5 QM-6 QM-7 Monzonite MT-1 MT-2 MT-4 MT-5 Minette 1000 100 10 1000 100 10 Rock/Chondrite Rock/Primitivemantle Minette Rb BaThNbTaKLaCeSrPNdZrHfSmEuTiDyYYbLu La CePrNdPmSmEuGdTbDyHoErTmYbLu 0.1 0.1 0.1 0.1

Fig. 3. Chondrite-normalized REE patterns and Primitive-mantle (PM) normalized spidergrams for monzonites (a,b) and minettes (c,d) of the Shahewan complex. Elements are arranged in the order of decreasing incompatibility from left to right for the spidergrams. The chondrite and primitive mantle values (in ppm) are fromSun and McDonough (1989).

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The zircon U–Pb age and K-feldspar40Ar/39Ar MDD modeling result of sample QM-2 show a two stage cooling history for the Shahewan complex: 17.7°C/Ma during the emplacement time (211 Ma to 180 Ma) and 7.7°C/Ma from 180 to150 Ma (Fig. 8). The average cooling rate of the Shahewan complex is11.5 °C/Ma, implying a fast exhumation process of it.

5. Discussion

5.1. Genesis of minette

The minettes generally have high MgO (max. 9.65 wt.%; Mg# = 70–80, Table 1), and compatible ele-ments in olivine and clinopyroxene such as Ni (max. 52 ppm, Table 1), and V (max. 127 ppm, Table 1), sug-gesting a magma source in the upper mantle. The mantle source mark in these minettes is also shown by the isoto-pic data. The weakly enriched initial87Sr/86Sr and slightly negative eNd(t) (Table 1,Fig. 4), close to the domain of the

depleted mantle in Fig. 4, favour a lithospheric mantle source. Although these dykes are also characterized by ‘‘crustal-like’’ trace element features, e.g., the enrichment in light rare earth (LREE) and other incompatible ele-ments, depletion in Nb, Ta, P, and Ti (Table 1, Fig. 3), these are widely explained to have originated from an enriched lithospheric mantle source, similar to those from Sulu–Dabie region and Jiaodong Penisula (Yang et al., 2005; Guo et al., 2004). The marked ‘negative’ anomalies of the high field strength elements, such as Nb, Ta, Ti and P in the primitive mantle-normalized element abundance plot (Fig. 3d), indicate that the magmas were derived from a source that was previously spiked in LILE and LREE by slab derived hydrous fluids and melts (Sun et al., 2002b), similar to those in the Sulu collisional belt (Guo et al., 2004).

The geochemical features such as the increasing of FeO, TiO2, V and Ni, the decreasing of Al2O3, SiO2with

decreasing MgO (Table 1, Fig. 2), suggest that the

mine-DM CH UR (87Sr/86Sr) i 3 Nd (T ) -20 -15 -10 -5 0 5 10 Monzonite Minette 0.70 0.71 0.72 0.73 0.74 0.75 0.76 0.77 Other granitoids in Qinling belt (~220-205M a) -5 -4 -3 -2 -1 0 0.704 0.705 0.706 0.707

Fig. 4. eNd(T) vs. (87Sr/86Sr)idiagram for the Shahewan complex. Other granites with the ages ranging between 214 and 209 Ma (Zhang, 1994) are also shown for comparison.

Ta ble 2 Zirco n U–Pb ana lytica l data of the Sh ahewan pluton (QM-2) Zirco n fractio n W eight (l g) U (pp m) Pb (ppm) Isotop ic rat ios Age (Ma ) 206 Pb/ 204 Pb 208 Pb/ 206 Pb 206 Pb/ 238 U2 r 207 Pb/ 235 U2 r 207 Pb/ 206 Pb 2 r 206 Pb/ 238 U 207 Pb/ 235 U 207 Pb/ 206 Pb (1) 40 502 31 101 0.2130 0.03305 41 0.2298 41 0.05043 61 210 210 215 (2) 35 657 26 531 0.2027 0.03331 41 0.2316 38 0.05042 51 211 212 215 (3) 30 759 30 655 0.1992 0.03380 42 0.2320 39 0.05041 52 212 212 214 (4) 30 469 35 74 0.2254 0.03302 48 0.2293 45 0.05036 88 209 210 212

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ttes may be the result of pyroxene-dominated fractionation of a mafic magma. Based on the data, therefore, this prim-itive magma should have low SiO2 (<52.18 wt.%), high

MgO (>9.65 wt.%), and elevated Mg-number (80), TiO2

(>0.62 wt.%), Ni (>52 ppm), V (>127 ppm), similar to the experiment melts from depleted peridotite (Falloon et al., 1988), indicating a dominantly refractory lithospheric man-tle contribution to the magma. The high K2O contents

(4.02–4.96 wt.%) of the minettes require a potassic phase in the source region. Melts in equilibrium with phlogopite are widely considered to have significantly higher Rb/Sr, Th/U and lower Ba/Rb values than those formed from amphibole-bearing sources (Furman and Graham, 1999; Mengel and Green, 1989; Turner et al., 1992, 1996; Wang et al., 2006). Low Rb/Sr (<0.3), Th/U (<3.5) and relatively high Ba/Rb (> 4.0) ratios strongly imply that the paren-tal magmas of the minettes formed through partial melting of an amphibole-bearing source. In addition, the high Zr/ Hf (20 30) ratios also suggest that amphibole were involved in the formation of the mafic magmas, which had been observed in the nearby Sulu region (Yang et al., 2005). Therefore, it can be concluded that the minettes were produced by crystal fractionation of a magma derived from partial melting of an amphibole-bearing, refractory lithospheric mantle source.

5.2. Genesis of monzonite

The monzonites have the similar initial 87Sr/86Sr and

143

Nd/144Nd ratios to those of the minettes, indicating they were derived from a common source. The weakly enriched initial 87Sr/86Sr and slightly negative eNd(t) (Table 1,

Fig. 4), close to the domain of mantle array (Fig. 4) but far from the lower crust (Qinling Group) and upper crust, favour a lithospheric mantle source contribution to the magma. In the major element vs. MgO diagrams (Fig. 2), the FeO, SiO2, Al2O3, V and Ni of the monzonites plot

along an extension of the minettes trend, implying that they

were the products of crystal fractionation of a mafic magma similar to the parental melts of the minettes. However, the absence of negative Eu anomalies (Fig. 3) in samples with lower initial87Sr/86Sr and143Nd/144Nd ratios indicates that the parental magmas have not experienced plagioclase frac-tionation. Recent studies on granitic rocks in a number of orogenic belts have identified broad positive arrays between Rb/Sr ratios of individual granite samples and the time-integrated Rb/Sr ratios of their source rocks inferred from their model neodymium ages (Kemp and Hawkesworth, 2003). This means that the Rb/Sr ratios of granites can reflect the Rb/Sr ratios of their source rocks. Therefore the source of the monzonites would have the low Rb/Sr ratios (<0.2), also implying an amphibole-bearing source.

The incompatible element contents (Table 1andFig. 3) are more typical of crustal-derived melts. From these chem-ical features the possibility remains that these monzonites were contaminated during transport through the continen-tal crust. However, the low initial 87Sr/86Sr and

143

Nd/144Nd ratios and absence of variation of these values with SiO2suggest that the parental melts of monzonites did

not assimilate crustal materials apparently, and kept a closed system crystal fractionation during magma ascent. The increase of CaO and the decrease of MgO with increas-ing SiO2contents (Table 1) indicate a pyroxene-dominated

fractionation process. Therefore, as discussed above, the minettes were derived from an enriched, refractory litho-spheric mantle. Thus, the monzonites could have been resulted by pyroxene-dominated fractionation from a parental mafic magma derived from lithospheric mantle. 5.3. Tectonic implications

The nondeformation of the Shahewan complex indicates that it had emplaced after the cessation of convergent deformation. It is common that orogenic granites widely occur in orogens (Turner et al., 1992), which are referred to as ‘‘postorogenic’’ suite. According to the comprehen-sive studies, orogenesis along the Shangdan suture zone terminated roughly by Middle Triassic (Sengor, 1985; Hsu¨ et al., 1987; Zhang et al., 1989, 1995; Reischmann et al., 1990; Huang and Wu, 1992; Kro¨ner et al., 1993; Mattauer et al., 1985; Xu et al., 2000; Ratschbacher et al., 2003). The metamorphic ages of Mianlue ophiolite (242–221 Ma, Li and Sun, 1996) and the blueschist (240 Ma, Yin and Nie, 1993; 232–216 Ma, Mattauer et al., 1985) suggest that the convergent deformation ended by 216–220 Ma in the eastern Qinling, quite similar to the ultrahigh-pressure metamorphic (UHP) rocks in the Dabie–Sulu belt (244–221 Ma) (Li et al., 2000; Chavagnac and Jahn, 1996; Hacker et al., 1998). Recently obtained U– Pb ages for the nondeformed granite suites in the south Qinling indicate that they were formed between 220 and 205 Ma (Sun et al., 2002a), immediately postdating the ces-sation of convergent deformation. These data verify the inference that the last collision along the Qinling belt hap-pened at the Late Triassic time (Meng and Zhang, 2000;

200 210 220 230 0.030 0.032 0.034 0.036 0.038 0.20 0.21 0.22 0.23 0.24 0.25 0.26 20 7 Pb /235U 20 6 Pb / 23 8 U Weighted Average age 211+/-2Ma

Fig. 5. Concordia diagram showing data of the U–Pb analyses of zircon fractions from the monzonites of the Shahewan complex. The data locate together on the Concordia curve giving a weighted average age of 211 ± 2 Ma.

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Table 3

Ar isotopic analyses on the biotite from MT-1 and K-feldspar from OM-2

T(°C) Cum.39Ar 40Ar* (%) 36Ar/39Ar 37Ar/39Ar 38Ar/39Ar 40Ar/39Ar 40Ar/36Ar Date (Ma) MT-1 biotite

650 .001 26.9 .9457E-01 .5617E-01 .3174E-01 .3778E+02 .3994E+03 102.2±1.8

730 .004 62.3 .4034E-01 .6732E-01 .2230E-01 .3199E+02 .7930E+03 203.0±7.7

800 .015 64.1 .3710E-01 .3988E-01 .2103E-01 .3055E+02 .8234E+03 198.3±1.4

850 .081 92.7 .5522E-02 .4403E-02 .1486E-01 .2241E+02 .4059E+04 209.8±1.4

900 .131 93.1 .5224E-02 .3821E-02 .1478E-01 .2228E+02 .4265E+04 209.3±1.4

950 .180 96.3 .2377E-02 .3279E-02 .1423E-01 .2148E+02 .9038E+04 209.7±1.4

1000 .249 98.2 .1306E-02 .2834E-02 .1399E-01 .2118E+02 .1622E+05 209.9±1.4

1050 .298 98.1 .1366E-02 .3248E-02 .1399E-01 .2118E+02 .1551E+05 209.7±1.4

1100 .314 98.6 .9733E-03 .4143E-02 .1380E-01 .2108E+02 .2165E+05 209.8±1.4

1150 .478 98.9 .7867E-03 .3341E-02 .1376E-01 .2099E+02 .2669E+05 209.6±1.4

1180 .639 98.8 .8165E-03 .3193E-02 .1386E-01 .2099E+02 .2570E+05 209.4±1.4

1220 .772 98.8 .8273E-03 .2893E-02 .1390E-01 .2100E+02 .2538E+05 209.5±1.4

1260 .860 98.8 .8704E-03 .3663E-02 .1393E-01 .2099E+02 .2412E+05 209.3±1.5

1300 .925 98.8 .8240E-03 .7955E-02 .1375E-01 .2098E+02 .2546E+05 209.3±1.4

1350 .988 98.8 .8400E-03 .7670E-02 .1372E-01 .2094E+02 .2493E+05 208.9±1.4

1500 1.000 98.6 .9530E-03 .1082E-01 .1378E-01 .2103E+02 .2206E+05 209.4±1.4

Sample mass = 25.3 mg

J-value = 0.00594060 ± 0.00004160 Integrated date = 209.2 ± 1.4 Ma QM-2 K-feldspar

450 .001 78.7 .3225E-01 .1759E+01 .5076E+00 .4438E+02 .1376E+04 238.6±85.8

450 .003 42.1 .3246E+00 .5989E+00 .2326E+00 .1659E+03 .5110E+03 449.3±15.1

500 .006 72.4 .1197E+00 .3180E+00 .1586E+00 .1279E+03 .1068E+04 573.2±16.6

500 .010 63.6 .5297E-01 .1737E+00 .8700E-01 .4296E+02 .8110E+03 188.8±12.4

550 .015 99.9 .1735E-04 .1169E+00 .7713E-01 .5077E+02 .2927E+07 336.5±4.7

550 .025 99.8 .9745E-03 .1569E+00 .3841E-01 .2484E+02 .2549E+05 170.6±2.4

600 .034 100.0 .9861E-05 .1380E+00 .3405E-01 .3141E+02 .3185E+07 215.4±8.2

600 .047 97.2 .2127E-02 .6715E-05 .2631E-01 .2218E+02 .1043E+05 150.5±4.1

650 .059 98.2 .1128E-01 .1297E+00 .2976E-01 .2799E+02 .2482E+04 171.3±4.5

650 .078 100.0 .4599E-05 .6505E-01 .1933E-01 .2277E+02 .4951E+07 158.6±2.5

700 .095 99.9 .9179E-03 .8016E-01 .2016E-01 .2538E+02 .2765E+05 174.2±2.6

700 .120 98.8 .1670E-02 .8519E-01 .1624E-01 .2310E+02 .1383E+05 157.5±0.4

750 .142 98.6 .1211E-02 .9478E-01 .1695E-01 .2438E+02 .2014E+05 167.0±2.1

750 .175 98.2 .1419E-02 .8442E-01 .1521E-01 .2379E+02 .1677E+05 162.6±0.9

800 .202 100.0 .3258E-05 .8515E-01 .1655E-01 .2469E+02 .7578E+07 171.4±1.6

800 .240 100.0 .2317E-05 .8594E-01 .1701E-01 .2458E+02 .1061E+08 170.7±0.6

850 .268 99.7 .2509E-03 .9486E-01 .1523E-01 .2521E+02 .1005E+06 174.4±1.3

850 .305 100.0 .2375E-05 .9477E-01 .1389E-01 .2538E+02 .1069E+08 176.0±0.4

875 .325 100.0 .4443E-05 .6152E-01 .1346E-01 .2554E+02 .5748E+07 177.0±2.6

900 .346 98.1 .1713E-02 .4222E-01 .1610E-01 .2563E+02 .1496E+05 174.3±0.5

925 .368 100.0 .3805E-05 .4875E-01 .1515E-01 .2578E+02 .6774E+07 178.6±1.0

950 .392 97.1 .2539E-02 .7769E-01 .1733E-01 .2545E+02 .1002E+05 171.5±1.7

975 .416 96.4 .2888E-02 .3149E-01 .1850E-01 .2534E+02 .8776E+04 170.1±0.9

1000 .441 98.8 .1059E-02 .1112E+00 .2039E-01 .2540E+02 .2399E+05 174.1±1.9

1000 .470 99.7 .1453E-03 .8714E-01 .1982E-01 .2501E+02 .1721E+06 173.3±2.3

1025 .489 100.0 .4679E-05 .6965E-01 .1725E-01 .2523E+02 .5392E+07 175.0±0.6

1025 .514 97.2 .2370E-02 .9579E-01 .1828E-01 .2519E+02 .1063E+05 170.1±0.7

1050 .531 99.7 .1903E-03 .2305E-01 .2075E-01 .2565E+02 .1348E+06 177.4±1.2

1050 .557 99.9 .3354E-05 .4237E-02 .1725E-01 .2570E+02 .7663E+07 178.1±1.1

1050 .358 100.0 .4424E-05 .5209E-01 .1514E-01 .2607E+02 .5892E+07 180.5±1.1

1050 .607 100.0 .2908E-05 .2908E-01 .1609E-01 .2628E+02 .9035E+07 181.9±1.0

1075 .630 100.0 .3842E-05 .2703E-01 .1444E-01 .2651E+02 .6902E+07 183.4±1.4

1075 .669 99.2 .8038E-03 .1997E-01 .1337E-01 .2655E+02 .3303E+05 182.1±1.4

1075 .699 100.0 .2947E-05 .1245E-01 .1379E-01 .2679E+02 .9093E+07 185.3±0.6

1075 .745 100.0 .1902E-05 .6907E-02 .1396E-01 .2669E+02 .1403E+08 184.6±1.2

1100 .782 99.9 .2330E-05 .2330E-05 .1400E-01 .2701E+02 .1159E+08 186.7±0.5

1100 .832 100.0 .1751E-05 .3566E-01 .1613E-01 .2708E+02 .1546E+08 187.2±0.6

1100 .904 100.0 .1208E-05 .1799E-01 .1487E-01 .2720E+02 .2251E+08 187.9±0.4

1100 .965 99.2 .8002E-03 .2574E-01 .1686E-01 .2933E+02 .3665E+05 200.4±0.7

1100 .987 99.9 .4018E-05 .4018E-05 .1799E-01 .3012E+02 .7495E+07 207.0±1.8

1150 .996 100.0 .9862E-05 .7524E-01 .1104E-01 .3164E+02 .3208E+07 216.8±3.8

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Sun et al., 2002c). Although the time lag between magmatic pulses does not necessarily imply any lag between the end of deformation and the first appearance of nondeformed granites, these data and the zircon U–Pb ages show that the Shahewan monzonites emplaced (211±2 Ma) 10 m.y. later than the collision of the Qinling–Dabie belt. There-fore, the Shahewan complex should be classified as ‘‘post-collisional’’ suite.

Petrological and geochemical data emphasize that the Shahewan complex was derived by partial melting of the enriched refractory lithospheric mantle source. They have geochemical features similar to postcollisional lavas emplaced in the other parts of the Tethyan orogenic belts, Table 3 (continued)

T(°C) Cum.39Ar 40Ar* (%) 36Ar/39Ar 37Ar/39Ar 38Ar/39Ar 40Ar/39Ar 40Ar/36Ar Date (Ma)

1200 1.00 98.2 .1978E-02 .1521E+00 .2074E-04 .3422E+02 .1730E+05 229.8±8.0

Sample mass = 45.8 mg

J-value = 0.00404072 ± 0.00000400 Integrated date = 182.6 ± 0.3 Ma

Note. Cum.39Ar denotes the cumulative of39Ar during the stepheating;40Ar*% means the percentage of radiogenic40Ar at every step.

Age (Ma) Cumlative 39Ar% 0 50 100 150 200 250 300 0 10 20 30 40 50 60 70 80 90 100 QM-2Ksp MT-1Bio Age (Ma) Cumlative 39Ar% 100 150 200 250 300 0 10 20 30 40 50 60 70 80 90 100 209.41 1.4Ma+-

Fig. 6. 40Ar/39Ar age spectra for biotite (a) from MT-1 and K-feldspar (b) from QM-2. The width of boxes of the spectra represents the relative fraction of

39Ar

kreleased in step heating and the height the error (2r) of the age of the corresponding step.

Table 4

Parameters (activation energy E, relative domain size, log (D0/q 2

), percentage of released gas (/)) used for K-feldspars’Arrhenius modeling

Domains E (kcal/mol) Relative domain size log (D0/q2) (s-1) / (39Ar%) 1 48.5 ± 1.6 0.00145 6.84 9.82 2 48.5 ± 1.6 0.00369 5.56 13.376 3 48.5 ± 1.6 0.03810 5.55 10.877 4 48.5 ± 1.6 0.03978 3.22 4.508 5 48.5 ± 1.6 0.03984 3.21 5.518 6 48.5 ± 1.6 0.04012 3.21 13.570 7 48.5 ± 1.6 0.24031 3.13 14.549 8 48.5 ± 1.6 0.80642 2.17 14.858 9 48.5 ± 1.6 1.00000 1.34 12.925

Fig. 7. Experimental and MDD modeling diagrams for QM-2 K-feldspar age spectrum: (a) Experimental and theoretical age spectra, (b) calculated cooling paths with a inset showing the 90% confident interval of the total distribution of cooling histories and the 90% confident interval of the median of the distribution.

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such as the Spanish lamproites (Nelson et al., 1986), Sulu ultrapotassic granitic rocks (Yang et al., 2005) and Tibetan shoshonitic rocks (Turner et al., 1996; Miller et al., 1999). The high-K magmatism is considered as a mark of postcol-lisional extensional setting (Bonin, 1990; Bonin et al., 1998; Turner et al., 1996).

The rapid cooling history obtained from the40Ar/39Ar thermochronology (Fig. 8) is inconsistent with a pure con-ductive cooling concluded by McDougall and Harrison (1999) and thus rather reflects an important denudation event undisturbed from 211 to 150 Ma, suggesting a fast exhumation of the Shahewan complex. Absence of sedi-mentation in the North Qinling offers support that the denudation kept going through the Triassic time. Although the collision between the South China Block (Yangzi Block) and South Qinling, having taken place during the Late Triassic time (Meng and Zhang, 2000; Sun et al., 2002c), affected the whole Qinling belt, the nondeformed extensive granite intrusions during the Late Triassic sug-gest that the convergent deformation was not dominant as discussed above. During a convergent deformation, the thickening of the crust results in increased gravitational potential energy and extensional buoyancy forces (England and Houseman, 1988, 1989; Sandiford and Powell, 1990; Turner et al., 1992, 1996). It follows that the extension and uplift will occur if the convergent plate driving forces are reduced or the internal buoyancy forces exceed the external driving forces. But the amount of uplift by this iso-static adjustment is only about 1–3 km (England and Mol-nar, 1990), not enough to exhumate the Shahewan complex to a shallow depth in the upper crust (convert 150°C to 3 to7 km below the earth surface based on the geothermal gradient of 20–50 °C/km supposedly). Therefore, the rapid exhumation of the Shahewan complex should result from different mechanics after the collision (see discussion later). Rapid uplift and extension during Cretaceous and Cenozoic times (Meng and Zhang, 2000) indicate that the uplift and extension occurred over a long time and was a

significant process in the North Qinling after the conver-gent deformation.

The rapid exhumation could cause the rapid pressure release of the emplaced magma from the overlying strata, this may provide an alternative rationale for formation of the rapakivi texture (K-felapar megacrystals mantled by plagioclase) within the monzonite of the Shahewan com-plex, although it can also be explained by the magma mix-ing (Wang et al., 2005). The calculation indicates that the decompression can produce paragenesis that can lead to all of the features of the rapakivi texture (Nekvasil, 1991). Several models have been proposed to explain the post-collisional magmatism, such as lithospheric delamination (Bird, 1979), convective removal of a thickened thermal boundary layer (Houseman et al., 1981) or the mantle root of the orogen (Turner et al., 1992, 1996), and the breakoff of an oceanic slab from the buoyant continental litho-sphere during subduction (Davies and von Blanckenburg, 1995). The breakoff model was employed to interpret the granitic intrusions in the South Qinling following the sub-duction of the South China (Sun et al., 2002a), supposing the breakoff of the slab happened at a very shallow depth which disturbed the asthenosphere greatly and led to the formation of the orogenic granitoids, because small frac-tions of K-rich melt can be produced by fluid-absent phengite dehydration-melting of metabasalts (flush melt-ing) (Schmidt and Poli, 2003). Zheng et al. (2003) argue that this mechanism can account for the Late Triassic potassic-ultrapotassic rocks in the Dabie–Sulu orogenic belt and thus classified them as the product of syn-exhuma-tion magmatism. Geochemical and Sr, Nd isotopic data indicate that the Shahewan complex was derived from an enriched lithospheric mantle. In this regard, part of the subducted South China lithospheric mantle and its overly-ing oceanic basaltic crust after the breakoff is plausibly responsible for generating the observed geochemical and isotopic features for both minettes and monzonites.

An alternative is thinning of the lithosphere and crustal extension induced by convective stability of a thickened mantle boundary layer (Houseman et al., 1981; Turner et al., 1992, 1996). Partial melting of an enriched refractory lithospheric mantle source from which the Shahewan com-plex was derived requires a sufficiently high temperature. The nondeformed magmatic suites of orogens seem to take the higher magmatic temperature than those deformed oro-genic suites (Turner et al., 1992). This means that a signif-icant perturbation in the thermal regime of the orogen is necessary. The cooling history of the Shahewan complex necessitates a considerable extent of uplift which is unlikely to be produced by isostatic response only after the cessa-tion of convergent deformacessa-tion. The conveccessa-tion mantle thinning (Houseman et al., 1981; Turner et al., 1992, 1996) may be a plausible means of attaining a temperature high enough for melting the lithosperic mantle and provid-ing tremendous uplift.

Thinning of the lithospheric mantle under the North Qinling orogen may be an automatic response to litho-Age (Ma) 200 100 100 300 400 500 600 150 200 Temperature (ºC) 700 800 900 250 K-fieldspar Ar/Ar MDD modelling Zircon U-Pb 7.7 C/Ma o 17.7 C/Ma o

Fig. 8. Compilation of the cooling history of the Shahewan complex. The values of cooling rates are shown.

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spheric thickening as a result from the collision. Prior to convective thinning of mantle lithosphere, the driving forces and buoyancy forces were in balance, the uplift was limited and the lithospheric mantle did not melt due to low heating. With the increase in degree of convective thinning of the mantle lithosphere, increase of the on potential energy of orogen would preclude further com-pression deformation and induce uplift and extension. Convective thinning of the lithospheric mantle brought previously cooler and insulated zones of the lithoshpheric mantle in direct contact with asthenospheric temperatures such that any metasomatized refractory lithospheric rocks underwent partial melting. Thinning or even removal of the root of the mantle lithosphere would result in very high Moho temperatures, and led heat into upper lithosphere, which triggered melting there beneath the Shahewan region.

Although the composition of the subcontinental mantle lithosphere beneath the Shahewan region cannot yet be quantitatively described, the geochemical data of Shahe-wan complex indicate that the mantle lithosphere was enriched. The degree of partial melting and compositional heterogeneities may account for this enrichment. It is likely that the mantle lithosphere is enriched or veined by small percentage (<1%) partial melts from the convective mantle that are enriched in light REEs and incompatible elements (Menzies and Murthy, 1980; O’Nions and McKenzie, 1988; Hawkesworth et al., 1990; Turner et al., 1992, 1996). Parts of a mantle lithosphere previously enriched or veined by such partial melts from the asthenosphere will have a lower solidus than the subjacent convective mantle. Resultant partial melt compositions will therefore be primitive, but with light-REE and incompatible elements enriched signa-tures (Turner et al., 1992, 1996).

Another source for the enrichment within the litho-spheric mantle beneath the Shahewan region may be from the subducted South China lithosphere during the Triassic time, which may happen near the boundary between conti-nental and oceanic lithospheres (Sun et al., 2002a). Although the process of continental subduction is charac-terized by the relative lack of fluids based on H and O iso-topic evidences (Zheng et al., 2003), the introduction metamorphic fluids derived from the decompression exso-lution from amphibole, phengite and clinopyroxene could metasomatize and vein the overlying lithosphere.

6. Conclusions

The Shahewan monzonite–minette complex is a product of postcollisional magmatism. The zircon U–Pb dating on the monzonite and40Ar/39Ar timing on the mafic minette dykes indicate that they emplaced coevally.40Ar/39Ar ther-mochronology on K-feldspar from the monzonite gives a two stage, rapid cooling history, suggesting a fast uplift after the cessation of the convergent deformation in the North Qinling during the Late Triassic time. Isotopic and geochemical data show that the complex was derived from

the partial melting of an enriched lithospheric mantle source. Two models, including the subduction model of the South China lithosphere beneath the North Qinling and the convective mantle thinning model, are assessed to construe the geodynamic setting of the Qinling orogenic belt during the Late Triassic time.

Acknowledgement

The authors are indebted to Dr Da-Ren Wen and Prof. Sun-Lin Chung from the Geology Department of National Taiwan University, for carefully carrying out Sr and Nd isotopic analyses. Damian Tootell is so kind for improving the English usage. We are very grateful to W.D. Sun and an anonymous reviewer for their thorough and helpful re-views that have significantly improved the manuscript. This work is supported by the Chinese National Natural Science Foundation (Grants No. 40673048).

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數據

Fig. 1. Geological sketch of the Qinling orogenic belt showing the locations of the granitoids and the Shahewan complex (compiled after Zhang, 1994; and Lu et al., 1999; and from new field observations)
Fig. 2. Various oxide plots: ((a) FeO; (b) Al 2 O 3 ; (c) TiO 2 ; and (e) SiO 2 in wt.%) and trace element plots (d, V, and f, Ni, in ppm) vs
Fig. 4. e Nd (T) vs. ( 87 Sr/ 86 Sr) i diagram for the Shahewan complex. Other granites with the ages ranging between 214 and 209 Ma (Zhang, 1994) are also shown for comparison.
Fig. 5. Concordia diagram showing data of the U–Pb analyses of zircon fractions from the monzonites of the Shahewan complex
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