西藏大竹区蛇绿岩中的钠长花岗岩

钠长花岗岩(Albite granite)是以钠长石和石英为主要成分,含有少量暗色矿物的花岗岩类岩石。它以不含(或偶含)钾长石为特征,故区别于大陆的钠长花岗岩、奥长花岗岩和花斑岩等花岗岩类岩石。

Study on the Tectonic Setting for the Ophiolites in Xigaze, Tibet

The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three-four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic-Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126-139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K-Ar age of amphibole in garnet amphibolite in the ophiolite melange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous. Research in petrology, petrological chemistry, mineralogy, and geochemistry of volcanic rocks and dikes of the Xigaze ophiolite indicate the following characteristics： （1） They are mainly composed of basalt, basaltic andesite, dolerite, and diabase and are characterized by high TiO2 （0.7-1.47%）, low MgO （mostly less than 8%）, and low SiO2 （mostly less than 53%）. （2） The volcanic rocks and dikes of the Xigaze ophiolite show light rare earth element （LREE）-depleted rare earth element （REE） patterns. （3） The spider diagrams of the volcanic rocks and dikes of the Xigaze ophiolite exhibit LILE depletion relative to high-field-strength element （HFSE） patterns with left oblique features. （4） No protogenetic olivine and clinoenstatite was detected. （5） Some dikes show low TiO2 and high MgO, in which a few of Cr-enriched spinels and a very few pseudomorphs of olivine, orthopyroxene can be seen. They show more distinctive affinity as boninitic rock and canbe classified to boninite series rock. The previously mentioned features of the volcanic rocks and dikes in the Xigaze ophiolite implies that these ophiolites formed in a mid-ocean ridge （MOR） in the earlier stage and than forearc extension of subduction initiation occurred once at the later stage of the evolution of the Xigaze ophiolite. The forearc extention caused further melting of the residue-depleted mantle, resulting in the formation of melts with lower TiO2 and higher MgO. These melts formed as dikes and intruded into the oceanic crust formed in the earlier stage, resulting in a close association of mid-ocean ridge basalt and the boninite rock of the Xigaze ophiolite.

Origin of the Zedang and Luobusa Ophiolites, Tibet

The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.

Petrological, geochemical and chronological constraints for the tectonic setting of the Dongco ophiolite in Tibet

The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O+ K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from +2.7―+5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ^(39)Ar/^(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.