新疆阿拉套山东部后碰撞岩浆活动的时代、地球化学性质及其对陆壳垂向增长的意义

Ages and geochemistry of the post-collisional granitic rocks from Eastern Alataw Mountains, Xinjiang, and implications for vertical crustal growth.

新疆阿拉套山南坡分布有大量花岗质岩浆岩,其中东部博乐地区的花岗质岩浆岩可以分为北部花岗岩、南部花岗岩、晚期岩脉和流纹岩四类。激光探针单颗粒锆石微区U-Pb同位素定年分析确定,北部岩体、南部岩体的侵位时代分别为 298．4±5．7Ma、292．4±4．9Ma,流纹岩喷出时代为270．7±6．5Ma,而岩脉的全岩Rb-Sr等时线年龄为285±13Ma(MSWD =0．52)。这四类花岗质岩石总体的地球化学性质相似,都属于准铝质到过铝质的后碰撞高钾钙碱性花岗岩,均以高的正εNd(T)值为特征。地球化学特征显示,这四类花岗质岩石具有同源岩浆分异演化的特点。从早到晚,前三类花岗岩中Ba、Sr、 P、Eu和Ti亏损程度增加,(La/Yb)n比值下降到接近1,岩浆温度逐渐降低,而εNd(T)值呈升高趋势。最晚形成的流纹岩中 Ba、Sr、P、Eu和Ti亏损程度减小,(La／Yb)n比值升高,岩浆温度再次升高,而εNd(T)值降低,ISr值升高,可能代表另一期岩浆活动的开始。这些花岗质岩石均具有高的εNd(T)值(2．10-5．18)和较年轻的Nd同位素模式年龄(TDM2=0．60-0．87 Ga), 显示该区花岗质岩石地幔物质组分含量较高。地幔物质的加入导致博乐地区大陆地壳的垂向增长．这与新疆北部300Ma 左右地壳垂向增长的过程是一致的。

Large amounts of granitic rocks intruded into Devonian and Carboniferous strata along the southern slope of the Alataw Mountains in Xinjiang province. The granitic rocks in eastern part of the Alataw Mountains can be recognized as 4 groups： northern granite, southern granite, dyke, and rhyolite. Zircon U-Pb dating by LA-ICP-MS indicates that the ages of northern granite, southern granite and rhyolite are 298.4±5.7 Ma, 292.4±4.9 Ma, and 270.7±6.5 Ma, respectively, and the whole-rock Rb-Sr isochron age for dyke group is 285 ± 13 Ma （ MSWD = 0.52）. They are all late Hercynian post-collisional granitic rocks. These granitic rocks are similar in geochemical features with large depletion of Eu （ Eu/Eu^＊ = 0. 005 - 0.15 ） , low （La/Yb）n ratios, and positive εNd （T）values ranging from 2. 10 to 5. 18, implying that they were formed from a common source. In the first three groups of granitic rocks, the depletion of Ba, Sr, P, Eu, and Ti increase, （La/Yb）, ratios reduce down to 1, zircon saturation temperatures decrease, but εNd（T）values show an increasing trend, as the ages become young. However, the late rhyolite displays the features similar to those of northern granite, with decreasing depletion of Ba, Sr, P, Eu, and Ti, increasing （ La/Yb）,, ratios and zircon saturation temperatures, lowest εNd （T） and highest ISr values, probably representing a new cycle of magmatism. The positive εNd （T） values and young two-stage Nd model ages （TDM2 = 0. 60 - 0. 87 Ga） imply high contributions of mantle-derived components to these granitic rocks, which resulted in vertical crustal growth in post-collisional setting. This is consistent with the process of post-collisional crustal growth in northern Xinjiang at around 300 Ma.