Inductively coupled plasma-mass spectrometry (ICP-MS)

Chapter 2 Research Methods

2.6 Inductively coupled plasma-mass spectrometry (ICP-MS)

The bulk rock trace elements analysis was determined using quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS, model Agilent 7500s) at the Geochemistry and Petrogenesis Lab, Department of Geosciences, National Taiwan University. The sample liquids used for ICP-MS were extracted using the procedure for extracting Sr and Nd isotopes. The measurements of the ICP-MS is corrected with standardsAGV-1, AGV-2, GSP-1, JB-1 and JG-1, and the precision was generally better than 5% (2σ) for most trace elements.

53 2.7 X-Ray Fluorescence Spectrometry (XRF)

Approximately 3 grams of rock powder from each sample was heated to 100°C for 3 hours to release ambient water from the sample and then heated to 900°C for 6 hours to oxidize and release molecular water. The net change of weight was recorded at each step and used to calculate the loss on ignition (LOI). 0.6 grams of rock powder was thoroughly mixed with 6 grams of Claisse^® lithium borates with lithium bromide flux (49.75% Li2B4O7, 49.75% LiBO2 with 0.5% LiBr). The mixed powder and flux was added to platinum (95%) and gold (5%) crucibles and fused at

~1200°C to make a glass bead using a Claisse^® M4 Fluxer (Pic. 16). The glass bead was loaded in the Panalytical Axios^mAX (Pic. 15) at the XRF Laboratory, Department of Earth Sciences, National Taiwan Normal University used for analysis. The calibration standards are United States Geological Survey (USGS) AGV-2 (andesite), BCR-2 (basalt, Columbia River), BHVO-2 (basalt, Hawaiian Volcanic Observatory), BIR-1a (Icelandic basalt), COQ-1 (carbonatite), DNC-1a (dolerite), DTS-2b (dunite, Twin Sisters Mountain), GSP-2 (granodiorite, Silver Plume, Colorado), SDC-1 (Mica Schist), SGR-1b (Green River Shale) and W-2a (diabase).

Picture 15. Panalytical Axios^mAX WDXRF.

Picture 16. Melting the powder and flux by Claisse^® M4 Fluxer.

Picture 17. Finished glass beads.

Chapter 3

Petrography

3.1 Petrography

The samples of this research were collected along Chia-Wu creek near Kuanshan, Taitung County. The collected specimen are several centimeters to 20 cm in diameter, and 8 out of 15 samples are used, including 7 peridotites and 1 gabbro.

The peridotites are darkish green in color with some olive green spots and composed primarily of serpentine. Most of the olive green spots are less than 0.5 cm in diameter.

Sample TD01001, TD01002, TD01003, TD01004 have some dark red color intermingled between the darkish green region whereas TD01007, TD01008 and TD01013 do not. Calcite veins cut through some rock samples (i.e. TD01001, TD01002, TD01003, TD01004 and TD01008). The gabbro sample (TD01014) is composed of ~55% mafic minerals (hornblende and oxide) and ~45% plagioclase and has an intergranular texture.

The peridotite samples are comprised mostly (≥ 90 vol%) of serpentine with minor amounts of orthopyroxene pseudomorphs (≥ 3 vol%) and spinel (≥ 1 vol%).

The original texture of the rock has been replaced but the serpentine suggests the rock was poikilitic. Small iron-oxide minerals form anastomosing bands which give the appearance of former grain boundaries. The oxide minerals are likely due to serpentinization of olivine. There are rare occurrences of party-altered (i.e.

pseudomorphic?) olivine which range in size from 0.5 mm to ~1 mm and still have discernable hexagonal to rounded shapes. Orthopyroxene pseudomorphs ranging in size from 0.5 mm to 3.5mm represent ~3% of the rock mode. The orthopyroxene pseudomorphs are commonly round and larger than the pseudomorphic olivine,

display a different relief from the serpentine and still have cleavage visible which makes them relatively easy to identify. Sub-hedral to anhedral spinel is probably the only preserved unaltered mineral in the rock. The spinel is between 0.5 and 2 mm in length and dark red in plain polarized light. Megnetite- and calcite-rich veins are common and likely the result of hydrothermal alteration of the original olivine and pyroxene. Samples TD01007 and TD01008 are relatively less altered than other peridotite samples and have slightly larger orthopyroxene pseudomorphs.

The gabbro is coarse grained and granular and consists of ~55% hornblende and ~45% of plagioclase with accessory quantities (i.e. < 1 vol%) of ilmenite and/or magnetite. The hornblende is light green under plain polarized light with second order interference colors under crossed polars (Pic. 21). The hornblende crystals are euhedral to sub-hedral and tend to be smaller (i.e. 0.1 mm) than the sub- to an-hedral plagioclase crystals (i.e. > 0.5 mm). The oxide minerals are small (i.e. < 0.05 mm), sub- to anhedral and commonly bordering plagioclase rather hornblende.

Picture 18. Optical microscope image of sample TD01001. Orthopyroxene pseudomorph with chrome spinel, iron oxide and serpentine in TD01001. (a) Plane polarized light (PPL), (b): Crossed-polars.

Picture 19. Optical microscope image of sample TD01001.

Calcite vein and orthopyroxene pseudomorph with iron oxide and serpentine in TD01001. (a) Plane polarized light (PPL), (b): Crossed-polars.

Picture 20. Optical microscope image of sample TD01002. Chromium spinel, calcite vein and serpentine in TD01002. (a) Plane polarized light (PPL), (b): Crossed-polars.

Picture 21. Optical microscope image of sample TD010014. Amphibole, plagioclase in TD01014. (a) Plane polarized light (PPL), (b) Crossed-polars.

62 3.2 SEM/EDS images

SEM images of the peridotites (i.e. TD01001-004, 007, 008 and 013) and gabbro (i.e. TD01014) are shown in picture 22 to 31. The serpentinized peridotites generally have similar textures (i.e. filled with ~95% of serpentine) but there are some differences in the level of alteration and formation of oxide minerals and calcite veins. Picture 22 and 23 are images of sample TD01001, which is mostly composed with serpentine (pale gray) but also with some Cr-spinel (white grains), iron oxide (thin white veinlet) and calcite (thick white vein in Pic. 23). Picture 24 shows the image of TD01002, which shows the same minerals as TD01001 but is much more enriched in calcite (light gray veinlet). The images of TD01003 (Pic. 25), TD01004 (Pic. 26), TD01007 (Pic. 27) are similar, with serpentine (pale gray), Cr-spinel (white grains) and little iron oxide (thin white veinlet). Sample TD01004 and TD01007 have fewer Cr-spinels than other samples. The images of TD01008 (Pic. 28) and TD01013 (Pic. 29) have more iron oxide than sample TD01001. The Cr-spinel grains in the peridotites are tens to few hundreds micrometers in length, and the bigger sized grains are euhedral or subhedral, while the middle and smaller sized crystals are subhedral or anhedral shape.

Picture 30 shows the textures of the gabbro sample TD01014, which is composed with about ~55% of hornblende and ~45% of plagioclase, and picture 31 shows the ilmenite in the sample.

Picture 22. Back-scattered electron (BSE) image of sample TD01001.

(serp. = serpentine, spl. = Cr-spinel, oxide = Fe oxide, same below.)

Picture 23. BSE image of sample TD01001.

(cal. = calcite, same below.)

Picture 24. BSE image of sample TD01002.

Picture 25. BSE image of sample TD01003.

Picture 26. BSE image of sample TD01004.

Picture 27. BSE image of sample TD01007.

Picture 28. BSE image of sample TD01008.

Picture 29. BSE image of sample TD01013.

Picture 30. BSE image of sample TD01014.

(Pl. = plagioclase, Hbl. = hornblende, same below.)

Picture 31. BSE image of sample TD01014.

(Ilm. = ilmenite).

Chapter 4

Results

4.1 Mineral Chemistry

4.1.1 Spinel from the ETO peridotites

The results of EPMA analysis are listed in appendix 1. The spinels are Cr-rich and classify as Cr-spinel. The Cr2O3 content ranges from a relatively low concentration of 35 wt% for TD01001, to an intermediate range between 38 and 41 wt% for TD01002, TD01003 and TD01004 and increases to a slightly higher concentration of 41 to 46 wt% for sample TD01007, TD01008 and TD01013. The mean Cr2O3 value is ~42 wt%. The mean aluminum content of all samples is ~26.5 wt%, while samples TD01001 and TD01003 have higher concentrations (31 and 29 wt%) and TD01008 and TD01013 has lower (25 wt%). The Cr and Al contents are used to calculate the Cr# (100Cr/ (Cr+Al)). The mean Cr number of all samples is

~51.6. Sample TD01001 has the lowest Cr number value (Cr# = 43), whereas all other samples have values >47. The highest Cr# is 54 from TD01008.

The MgO content of the spinels are relatively constant at ~15% but their overall range is from 12% to 17%. TD01001 and TD01003 have higher values than average, mostly above 15.5 wt% and up to 16.5 wt%. Except few analyses, the value of TD01004 and TD01007 concentrate at 14 to 15.5 wt%. The value of sample TD01008 and TD01013 are slightly lower than others, between 12.5 and 14.5 wt%.

Total iron content (detected as FeO) counts for all samples between 13 and 17 wt%.

The ratio of Mg and Fe, calculated as the Mg# (100Mg/ (Mg+Fe)) ranges between

60 and 66. The concentration of other elements in the analysis (SiO2, TiO2, MnO, NiO, CaO, Na2O and K2O) are lower than 0.5 wt%.

4.1.2 Gabbro Mineral Chemistry

The gabbro mainly consists of three minerals, hornblende, plagioclase and ilmenite. The EPMA results of hornblende give the SiO2 content between 44 and 49 wt%, FeO content between 16.5 and 19 wt%, MgO content between 10.5 and 14 wt%, CaO content between 9.5 and 11 wt%, Al2O3 content between 4.5 and 7.5 wt%, TiO2

content and Na2O content below 2 wt% and MnO and K2O under 0.2 wt%. The SiO2

content in plagioclase composition range between 64 and 68 wt%, Al2O3 between 19.5 and 20.5 wt%, CaO around 1 wt%. There is around 10 to 11 wt% Na2O in the plagioclase, while there’s merely no K2O in it (less than 0.2 wt%). And other elements are also close to detection limits. The ilmenite have ~47.5 to 50.7 wt % FeO and 47.9 to 49.3 wt% TiO2 with other element concentrations < 1 wt%.

70 4.2 In situ zircon U/Pb age dating result

Picture 32 to 34 show the CL images of the 20 zircons separated from the gabbro TD01014 used for LA-ICP-MS analysis. The grain size of the zircons are

~100 to ~250 μm in diameter, and most of the grains are broken. Some of the zircons have obvious zoning while some are more complicated.

The full U/Pb analysis data are shown in appendix 2. The average ²⁰⁶Pb/²³⁸U detection ratio is 0.0022 ± 0.00014, while the average ²⁰⁷Pb/²³⁵U detection ratio is 0.0134 ± 0.00361. The Concordia diagram, constructed using Isoplot 3.0 (Ludwig, 2003), is shown in figure 11 with 2 error ellipses of the individual spot analyses.

The weighted mean ²⁰⁶Pb/²³⁸U age is 14.1 ± 0.4 Ma on 20 individual zircon crystals and the MSWD (i.e. mean square of weighted deviation) is 4.2. Due to the young age of these zircons, the ²⁰⁷Pb/²³⁵U age has a larger uncertainty and thus is less meaning in this research.

Picture 32. Zircon sample CL image and LA-ICP-MS testing spot.

(The individual zircon age is listed in the picture, same below.)

Picture 33. Zircon sample CL image and LA-ICP-MS testing spot.

Picture 34. Zircon sample CL image and LA-ICP-MS testing spot.

Figure 11. ²⁰⁶Pb/²³⁸U - ²⁰⁷Pb/²³⁵U concordia plot.

0.000 0.004 0.008 0.012 0.016 0.020 0.024 0.028

U-Pb Concordia Age

Figure 12. ²⁰⁶Pb/²³⁸U mean age.

9 10 11 12 13 14 15 16 17 18

206

Pb/

²³⁸

U Age

Mean = 14.13 ± 0.37, 95% conf.

N = 20

data-point error symbols are 2

4.3 Trace elements, Rare Earth Elements (REEs) and Sr/Nd isotopes

The trace elements in the peridotite samples are quite depleted except the transitional metals (i.e. Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn). The concentration of the transition metals range from several ppm (e.g. Sc) to tens ppm (e.g. Ti, V, Zn) to hundreds ppm (Mn, Co) to thousands ppm (e.g. Cr, Ni). Sr concentration in TD01002 (~219 ppm) is several times higher than other samples (0 ~ 43 ppm). Sample TD01004, though still quite depleted (< 3 ppm), is more abundant in other trace elements (i.e. Ga, Rb, Y, Zr, Nb, Cs, Ba, Hf, Ta, W, Pb, Th) than other samples (< 1 ppm or under detection).

The rare earth elements (REEs) of all serpentinized peridotites except TD01004 are quite depleted (< 0.1 ppm). TD01004 has much higher concentration in REEs than other samples (~ 10 times). These serpentinized peridotites have a slight U-shape pattern (i.e. slightly higher LREE and HREE) and a negative Eu anomaly.

If normalized to chondrite values (McDonough and Sun, 1995), the REEs concentration except TD01004 are about 0.1 times of C1 chondrite concentration, while TD01004 is 1 to 2 times chondrite concentration.

The measurement of ⁸⁷Sr/⁸⁶Sr isotope ratio of four peridotite samples (i.e.

TD1001, TD01002, TD01003, and TD01004) ranges between 0.70883 and 0.70908.

The initial ⁸⁷Sr/⁸⁶Sr values cannot be calculated due to the extremely low Rb content (i.e. below detection limit) but given their likely age (i.e. ~14 Ma) the measured value will not be significantly different from the initial values (i.e. ± 0.00001). The Sm and Nd concentration of most peridotites is too low to measure the ¹⁴³Nd/¹⁴⁴Nd ratio;

however, sample TD1004 has sufficient concentration and has a measured

143Nd/¹⁴⁴Nd ratio of 0.513106. The initial ratio based on the age of the gabbro is 0.513083 and corresponds to an Nd(T) value of +9.1 using a CHURtoday value of 0.512638.

Gabbro sample TD01014 has a wide range of transitional metals, which is abundant in Ti (~15040 ppm) and V (~860 ppm) while depleted in Cr (~6 ppm) and Cu (~8 ppm). The REEs concentrations in TD01014 are generally about 10 times more abundant to the chondrite concentration, and it has a similar pattern as N-type (normal) MORB. The measured ⁸⁷Sr/⁸⁶Sr isotope ratio of 0.70503 corresponds to an ISr value of 0.70501. The measured 143Nd/144Nd isotope ratio is 0.513226 with an

Nd(T) value of +11.4.

Trace elements concentration ( in ppm)

TD01001 TD01002 TD01003 TD01004 TD01004-2 TD01007 TD01008 TD01013 TD01014

P 66 78 73 84 114 55 59 61 260

Table 2. Trace elements concentration of the East Taiwan Ophiolite.

REE concentration ( in ppm)

Chondrite TD01001 TD01002 TD01003 TD01004 TD01004-2 TD01007 TD01008 TD01013 TD01014

La 0.237 0.001 0.001 0.100 0.300 0.500 0.001 0.100 0.001 2.400

TD01001 TD01002 TD01003 TD01004 TD01004-2 TD01007 TD01008 TD01013 TD01014

La 0.004 0.004 0.422 1.266 2.110 0.004 0.422 0.004 10.127 normalization value based on the CI chondrite from McDonough and Sun (1995).

Figure 13. Sample REE concentration spider diagram. (Model by Sun and McDonough, 1989).

Figure 14. Sample trace elements spider diagram (Model by McDonough and Sun, 1995).

Sample No. Rock Type Rb (ppm) Sr (ppm) ⁸⁷Sr/⁸⁶Sr Sm (ppm) Nd (ppm) ¹⁴³Nd/¹⁴⁴Nd εNd(T)

TD01001 Peridotite 0 43 0.708891 ± 9 0 0 - -

TD01002 Peridotite 0 219 0.708833 ± 7 0 0 - -

TD01003 Peridotite 0 9 0.709076 ± 8 0 0 - -

TD01004 Peridotite 0 20 0.709035 ± 7 0.3 0.83 0.513106 ± 6 + 9.1

TD01007 Peridotite 0 0 - 0 0 - -

TD01008 Peridotite 0 0 - 0 0 - -

TD01013 Peridotite 0 0 - 0 0 - -

TD01014 Gabbro 3 219 0.705030 ± 7 3.1 8.51 0.513226 ± 6 + 11.4

Table 4. Sr-Nd isotopic ratio of the peridotite and gabbro.

83 4.4 Whole rock geochemistry

The peridotite samples have 37.9 to 39.9 wt% of SiO2, 35.8 to 38.5 wt% of MgO, 6.7 to 8.2 wt% Fe2O3, and < 1.0 wt% Al2O. TiO2, MnO, Na2O, K2O, P2O5

contents are close to the detection limit and commonly < 0.01 wt%. CaO content is more variable and is likely due to the presence of secondary calcite veins in the rock (c.f. Pic. 19 and 20). Samples TD01001, TD01002, TD01003 and TD01004 have CaO content > 0.5 wt% whereas samples TD01007, TD01008 and TD01013 have ≤ 0.05 wt%. The Mg# (100 Mg²⁺/ (Mg²⁺ + Fe²⁺)) of the serpentinized peridotite samples are between 90 and 92. The loss on ignition (LOI) of the peridotites is very high (i.e. 12 wt% to 13 wt%) and is consistent with highly serpentinized nature of the rocks. The gabbro has 48.4 wt% of SiO2, 2.6 wt% TiO2, 12.1 wt% Al2O3, 18.1 wt% Fe2O3, 0.2 wt% MnO, 5.9 wt% MgO, 8.1 wt% CaO, 3.6 wt% Na2O, 0.3 wt%

K2O, 0.1 wt% P2O5 and a LOI value of 0.9 wt%.

Content (wt %) TD01001 TD01002 TD01003 TD01004 TD01007 TD01008 TD01013 TD01014

Table 5. Bulk rock composition of the serpentinized peridotites (TD01001 to TD01013) and hornblende gabbro sample (TD01014).

Chapter 5

Discussion

5.1 Age of the East Taiwan Ophiolite

The first attempt to determine the age of the ETO was made by Huang et al.

(1979). Their study identified foraminifera species Sphenolithus heteromorphus and Calcidiscus macintyrei within red shales from nanofossil zone NN5 that constrains

the deposition age of the sediments to lower and middle Miocene (Fig. 15).

The first radioisotopic ages were reported by Jahn (1986) on two types of basaltic rock (i.e. glass and crystalline), plagiogranite and gabbro using the K-Ar method. The results produced four different ages: 33 ± 5 (plagiogranite), 14.6 ± 0.4 Ma (basaltic glass), 11 ± 4 Ma (gabbro) and 8.1 ± 0.9 Ma (crystalline basalt).

Consequently, Jahn (1986) interpreted the likely age of the ETO to be 14.6 ± 0.4 Ma based on mutual consistency with the paleontological interpretations of Huang et al.

(1979).

Recently, U-Pb age dates were reported by Shao (2015) on gabbro and diorite from the north branch of the Chiawu creek. The concordant ages of the gabbros are

~17.5 Ma (i.e. 17.5 ± 0.2 and 17.4 ± 0.2) whereas the diorites are 14.3 ± 0.5 Ma and plagiogranites are 14.1 ± 0.2 Ma. Shao (2015) did not provide an explanation for the 3 million years gap between the ages of the gabbros and the diorites. The mean zircon U-Pb date from the hornblende gabbro (i.e. 14.1 ± 0.4 Ma) in this study is within error of the K-Ar age of the basaltic glass reported by Jahn (1986), the diorite group of Shao (2015) and interpreted age of the red shale layers suggesting that the magmatism in the ETO was active during Langhian stage (within NN5) of the

Miocene. The radiometric age of the rocks from the ETO is as a reference of the duration of the active magmatism of the South China Sea.

Figure 15. The foraminifera timescale NN2 to NN6 correspond to the geological timescale.

(Timescale data from Mandur, 2009 and Marzouk, 2009)

5.2 Rare earth elements, trace elements and isotopic composition of the ETO mantle

Rare earth elements (REEs) of the rock shows the characteristics of the original composition of the magma and the surrounding environment during the rock forming process. In this research, the REEs of all the samples were analyzed, but several elements in the serpentinized peridotite samples are too low and cannot be detected, for instance, La, Ce and Lu. The REE concentration abundance is very low in peridotites (i.e. 0.001-0.1 ppm, or ~ 0.01-0.5 times of chondrite content) except sample TD01004. In the chondrite-normalized REE pattern graph (Fig. 16), the serpentinized peridotites show U shapes, where LREE and HREE are slightly abundant. The U-shaped REE patterns have been reported from harzburgite and dunite and associated boninite in several Phanerozoic ophiolites, including the Izu-Bonin-Mariana, Troodos, Urals, New Caledonia, Trinity, Pyrenean, Betts Cove, and Cuban ophiolites (Jahn, 1986; Gruau et al., 1998; Zhou et al., 2005; Kapsiotis, 2013).

The negative Eu anomalies are believed to be irrelevant to their magmatic processes;

instead, they could be induced by alteration/serpentinization (Fig. 16).

The REE patterns of the hornblende gabbro, on the other hand, are sub-parallel those of the peridotites at higher concentration levels. The slight depletion of LREE in hornblende gabbro (i.e. La/Yb ≦ 1) is very similar to of N-MORB (Fig. 17).

Figure 16. REE pattern of the peridotites show slightly U-shape and negative Eu anomaly.

Figure 17. Hornblende gabbro TD01014 REE pattern shows the same pattern of N-MORB.

The early studies have reported anomalously high ⁸⁷Sr/⁸⁶Sr values in serpentinized peridotite and concluded that they were genetically unrelated to oceanic crust (Bonatti et al., 1970; Menzies and Murthy, 1978); however, more recent studies in abyssal peridotites and in the plagioclase peridotite of the Zabargad Island (Red Sea) demonstrated that the high ⁸⁷Sr/⁸⁶Sr values measured in oceanic peridotites may result from seawater alteration processes (Kempton and Stephens, 1997; Gruau et al., 1998). Information conveyed by mantle xenoliths indicates that stable sub-continental lithosphere is dominated by refractory peridotites that are enriched in HIE and LREE and often have acquired an enriched isotopic signature. A striking feature of several orogenic peridotites is the existence of a negative correlation between Nd–

Sr isotopic enrichment and peridotite fertility. Bodinier et al. (1991) and Downes et al. (1991) suggest the involving of enriched melts in porous-flow channels would interact with the peridotites and produce more enriched compositions. At Lanzo, this process is tentatively ascribed to a ‘‘pre-rift’’ stage of the opening of the Liguro-Piemontese ocean basin (Bodinier et al., 1991). Secondary alteration processes are strongly responsible for element deviations, particularly the strong depletion of calcium and sodium relative to aluminum in some abyssal peridotites, as well as part of the scattered variations of silicon, magnesium, calcium, and sodium. Serpentinized, orogenic and ophiolitic peridotites may be strongly depleted in calcium and sodium as well.

Figure 18. Initial ⁸⁷Sr/⁸⁶Sr vs. εNd value.

5.3 The tectonic setting of the East Taiwan Ophiolite

The nature and tectonic setting of the East Taiwan Ophiolite is a topic that is still under debate (Suppe et al., 1981; Jahn, 1986; Chung and Sun, 1992; Shao, 2015).

Suppe et al. (1981) studied the stratigraphy of the Lichi formation of the Coastal Range of East Taiwan and suggested that the East Taiwan Ophiolite is a submarine scree deposit consisting of angular mafic and ultramafic plutonic blocks that formed at a ‘leaky’ transform fault offsetting (Liou et al., 1977, Suppe et al., 1981). Jahn (1986) presented geochemical and isotopic data of glassy basalts, gabbros and plagiogranites and considered that the East Taiwan Ophiolite is derived from a spreading ridge of an open ocean or marginal basin. Furthermore, Chung and Sun (1992) proposed that the East Taiwan Ophiolite formed at a slow spreading ridge environment. Shao (2015), however, compared the Hf isotope of gabbros, diorites and plagiogranites of the East Taiwan Ophiolite to similar rocks from other ophiolites and proposed that the East Taiwan Ophiolite is probably the result of upwelling of a fore-arc subduction zone.

The rock sequence of East Taiwan Ophiolite is described by previous studies (Liou et al., 1977, Liou and Ernst, 1979, Suppe et al., 1981, Chung and Sun, 1992) (Fig. 19). The red shale layers between plutonic rocks and the extrusive rocks indicate that it was exposed under the carbonate compensation depth (CCD) for a period of time, perhaps a deep ocean setting at great distance from land. If this is the case, a mid-ocean ridge setting seems to be more promising than the original ‘leaky olistostrome’ interpretation of Suppe et al. (1981). On the other hand the absence of boninite, an important mafic volcanic rock that is rich in both Mg and Si and

commonly found at subduction zones, suggests the East Taiwan Ophiolite is unlikely to related to a suprasubduction zone (SSZ) type ophiolite or to any subduction-related setting. The relatively high εNd(T) values of the gabbroic ETO ophiolitic rocks (i.e. >

+9) also suggests that the fore-arc subduction interpretation is vulnerable

The bulk rock geochemistry and mineral geochemistry of peridotites can be used to distinguish between different tectonic settings (Dick and Bullen, 1984;

Bonatii and Michael, 1989; Arai, 1992, 1994; Kamenetsky et al., 2001; Hebert et al., 2003). Some major and trace elements such as Mg, Cr and Al content, that are

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