塔什库尔干新生代碱性杂岩的地球化学特征及岩石成因  

Petrogenesis and geochemistry of Cenozoic Taxkorgan alkalic complex and its geological significance.

作　　者：柯珊[1] 莫宣学[1] 罗照华[1] 詹华明[2] 梁涛[2] 李莉[3] 李文韬[2] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083 [2]中国地质大学地球科学与资源学院,北京100083 [3]中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石学报》2006年第4期905-915,共11页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(2002CB412603;2002CB412600);国家自然科学基金(40272033;40472038);国土资源部青藏研究计划重点项目(2003009)资助成果。

摘　　要：帕米尔构造结是青藏高原构造挤压最强烈、地壳短缩量最大的地区之一。位于帕米尔构造结中东部的塔什库尔干碱性杂岩是区内最大的新生代碱性杂岩体,由苦子干正长岩岩体和卡日巴生碱性-偏碱性花岗岩岩体组成。霓辉石正长岩和正长花岗岩是苦子干碱性岩体的主要岩石类型。根据锆石SHRIMP定年,获得苦子干岩体正长岩和正长花岗岩的岩浆锆石年龄分别为11．1±0．3Ma和11．3±0．6Ma。苦子干岩体具富钾(平均6．22%)富钙的特点,属于钾质(K_2O/Na_2O＞1)碱性花岗岩类。各类岩石的稀土元素总量很高,强烈富集LREE,(La/Yb)_N比值(88～21)高,Eu异常不明显,表明岩浆来自斜长石不稳定区,而且源区有石榴石残留,相当于榴辉岩矿物组合。微量元素分析表明,岩石具有高Ba、Sr、Sr/Y比值的特征,同时富集大离子亲石元素LILE,相对贫HREE、HFSE,出现明显的Nb、Ta、Ti负异常,后者系源区残留有金红石的有力证据。根据斜长石、石榴石和金红石实验岩石学的约束,结合Sr、Nd、Pb同位素的特征,表明苦子干岩体来源于源区为榴辉岩相的加厚镁铁质下地壳,地壳厚度至少大于50km,且源区可能受到了来自俯冲带流体的影响。Pamir syntax is one of the areas which have the most strongly tectonic convergence and largest crustal shortening in Tibetan Plateau. Taxkorgan complex, mainly composed of Kuzigan alkaline syenites and Karibasheng subalkaline granitoids, is the largest Cenozoic alkaline complex in the Pamirs region. Aegirine-augite syenite and syenogranite constitute the Kuzigan alkaline complex. We obtain 11. 1 ±0. 3Ma of Kuganzi syenites and 11. 3 ±0. 6Ma of syenogranite by using SHRIMP U-Pb zircon dating. Being rich in potassium, alkali and calcium, Kuzigan complex is a series of potassic （K2O/Na2O 〉 1 ） and alkalic granitoids. This complex is strongly rich in LREE and has high Y. REE content, and （La/Yb）N （88 - 21 ） ratio without Eu negative anomaly. These features suggest that the magma came from the source that had no plagioclase but had garnet in its melting residue. Besides, this complex is characterized by high Ba, Sr, Sr/Y ratio, LILE and relatively low HREE, HFSE with obviously negative Nb, Ta, Ti anomalies resulted from the rutile in the residue. Based on the stability of pressure and temperature of plagioclase, garnet and rutile, and combined with the isotope characteristics, we suggest that this alkaline intrusion was the product of the partial melting of eclogite at the base of the thickened crust, which may be affected by fluids from the subducted zone. The age of the alkaline complex and the characteristics of its source region show that the crust of this region had been thickened at 11Ma, with a curst thickness over 50km.

关 键 词：塔什库尔干 正长岩 花岗岩 帕米尔 青藏高原 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]