东天山印支早期尾亚石英正长岩:成岩作用及成岩意义

Weiya quartz syenite in early Indosinina from eastern Tianshan Mountains:petrogenesis and tectonic implications.

前人对东天古生代的构造-热事件作了大量研究,得出了许多方面的重要认识,然对东天山印支期的构造演化目前仍不清楚。本文以印支早期的尾亚石英正长岩为例,通过其岩相学及地球化学特征研究,来探讨该岩体的成岩机制及成岩意义。尾亚石英正长岩主要组成矿物为石英(5%～10%)、斜长石(15%～20%,An16～18)和钾长石(60%～65%),含少量铁质黑云母(5%)和钙质角闪石(≈5%),不舍橄榄石和辉石。该岩石SiO_2含量为62．67%～66．22%,Na_2O+K_2O(10．25%～11．00%)和TiO_2(0．45%～0．70%)含量以及K_2O/Na_2O(0．85～1．60)、FeO^T/Mgo(5．7～15．86)和CaO/Na_2O(0．37～0．68)比值高,而Mgo(0．34%～0．43%)含量以及Al_2O_3/TiO_2(23．77～38．27)比值低,偏铝(ASI 0．89～0．98),富集Rb、Ba和Pb等大离子亲石元素(LILE)和U、Th、Zr和Hf等高场强元素(HFSE),亏损Sr等大离子亲石元素及Nb和Ti等高场强元素。ΣLREE,ΣHREE和ΣREE分别为76．00×10^(-6)～139．54×10^(-6),13．54×10^(-6)～22．67×10^(-6)和96．57×10^(-6)～153．08×10^(-6), (La/sm)_N,(Gd/Yb)_N,(La/Yb)_N分别为1．86～4．78,0．73～1．64和2．10～9．64,δEu为0．94～2．83,具有稀土总量较低、轻稀土比重稀土分馏大、轻、重稀土分馏不明显以及正Eu异常(0．94～2．83)的特征。岩相学及地球化学特征研究表明,尾亚石英正长岩不是幔源岩浆分异的产物,而是直接来自于壳源。通过对比分析其它正长岩地球化学特征并结合前人实验岩石学研究,得出了尾亚石英正长岩是在950℃～1100℃、15～20 kbar条件下,由陆内长英质麻粒岩在加厚陆壳底部发生增温和减压熔融的产物的认识。这样的成岩条件可能与特提斯洋于海西晚期—印支早期向北俯冲所引发的天山地区强烈的陆内A-型俯冲及随后的伸展作用有关。本文结果表明,东天山地区在二叠纪晚期到三叠纪早期可能发生了从受古亚洲洋俯冲-碰撞体系影响为主转变为受古特提斯洋构造域影响为主的重大事件。

Voluminous literature has resulted from study of the geology and tectonic environments of the eastern Tianshan Mountains in Paleozoic, whereas tectonic evolution of the eastern Tianshan Mountains in Indosinian has not been well understood. In this contribution, we report the petrography and geochemical data for quartz syenite within the Weiya complex from eastern segment of the Middle Tianshan Mountains to constrain petrogenesis and discuss tectonic implications. Petrographically, the rock is composed of quartz （5% - 10% ） , plagioclase （ 15% -20% ,An16 - 18） and K-feldspar （60% -65% ） with minor Fe-rich biotite （5%） and calcichornblende （ ≈5% ）. No pyroxene or olivine has been found. Geochemically, SiO2 content of the syenite varies between 62.67% and 66.22 %. It has high content of Na2O ＋ K2O （ 10.25% - 11.00% ） and TiO2 （0.45% - 0.70% ）, high ratios of K2O/Na2O （0.85 - 1.60） ,FeO^T/MgO （5.7 - 15.86） and CaO/Na2O （0.37 - 0.68 ）, whereas the content of MgO （0.34% - 0.43% ） and ratio of Al2 O3/TiO2 （23.77 - 38.27 ） are low. The rock is metaluminous with aluminium saturation indices （ASI） of 0.89 - 0.98. The Weiya quartz syenite is characterized by enrichment in LILE such as Rb, Ba and Pb and in HFSE such as U, Th, Zr, Hf, but depletion in LILE such as Sr and in HFSE such as Nb and Ti. Contents of ∑REE in the rock are low and vary from 96.57 to 153.08 （ × 10^-6 ）. Contents of ∑LREE and∑HREE in this rock are 76.00 × 10^-6-139.54 × 10^-6 and 13.54 × 10^-6 -22.67× 10^-6, respectively; the ratios of （La/Sm）N, （ Gd/Yb）N, （ La/Yb）N are 1. 86 - 4. 78,0. 73 - 1.64 and 2. 10 - 9. 64, respectively, indicating that fractionation between LREE is larger than that between HREE while there is no significant fractionation between LREE and HREE. The δEu values varying from 0.94 to 2. 83 indicate that Weiya quartz syenite is characteristics of positive Eu anomalies. Studies in petrography and geochemistry of the Weiya quartz syenite indicate that the rock was derived from crust-rather than mantle-origin. Based on the results of experimental petrology and geochemical comparison with other syenite bodies, it has been concluded that the Weiya quartz syenite, formed at high temperature （950℃ -1100℃ ） and pressure （15 -20 kbar）, was derived from dehydration melting of felsic granulites at the bottom of a thickened continental crust as a consequence of temperature rising and decompression. Magma generation could have been induced by northward subduction of the Tethys Ocean from late Hercynian to early Indosinian which triggered intense intra-continental A-type subduction followed by extension in the Tianshan Mountains. It is supposed that a geodynamic conversion from the Paleo-Asian ocean subduction-collision system to the Paleo-Thethys ocean regime took place during the late Permian to early Triassic time in the eastern Tianshan Mountains.