东秦岭北部富碱侵入岩带岩石地球化学特征及构造意义

THE GEOCHEMISTRY OF THE ALKALI-RICH INTRUSIVE ROCKS IN THE EAST QINLING,CENTRAL CHINA

在东秦岭北部,富碱侵入岩的侵位与空间分布受同一个区域构造带(华北陆块南缘)控制,构成一个区域性的富碱岩浆岩带。根据岩石学和岩石化学研究,岩石类型主要分为碱性正长岩、碱性花岗岩和石英正长岩三大类。富碱岩浆岩带自北而南可以划分为3个亚带:北部碱性正长岩亚带,中部碱性花岗岩亚带,南部石英正长岩亚带。3个亚带富碱岩浆在化学成分方面虽有差异,但都具有富碱高钾特征,ALK=9～15,ω(K_2O)为5％～15％,ω(K_2O)/ω(Na_2O)=1.26～8.30。岩石中的暗色矿物辉石类主要为霓辉石和霓石,角闪石类主要为钙质浅闪石、阳起石质闪石、镁质角闪石、钠钙质绿闪石,黑云母类主要为铁云母和金云母。岩石中的长石类主要为K-Na系列富K端员的微斜长石和最大微斜长石,少量钠长石,极少Na-Ca系列的斜长石。霞石类主要为钾霞石和钙霞石。根据岩石地球化学研究,有以下显示:(1)REE总量200～1100μg/g,LREE/HREE比值4～15,δEu表现为无Eu异常或轻微正负Eu异常,(La/Yb)_n值多为10～30;(2)部分岩石类型富集大离子亲石元素,不相容元素分布模式曲线总斜率为负,Ba,Nb,Zr具明显负异常,表明他们具有大陆裂谷碱性花岗岩的特征;(3)岩石Nd、Sr和Pb同位素的研究表明,富碱侵入体的源区应是以下地壳为主,带入少量地幔和上地壳物质。

The alkali-enriched intrusive rocks are mainly located in the north of East Qinling, central China, and their occurrences are controlled by a regional tectonic zone. Based upon systematic petro-graphic and geochemical studies, the intrusive rocks are divided into three types: (1) The alkaline rock type, that is syenitoid containing foid or alkaline dark minerals; (2) The alkaline granitoid including arfvedsonite granite and assemblage moyite; (3) Quartz syenite including quartz syenite, nordmarkite and granosyenite (porphyry) , which took alkaline feldspars as the dominate factors. Based upon systematic petrographic and dark minerals chemistry studies, we divided the dark minerals into three types, i. e. , alkaline augite including aegirine and aegirin-augite, alkali amphibole including calc-edenite, actinol-ite-amphibole, magnesio-hornblende, and sodium-calcium taramite, and biotite including phlogopite and annite. It is demonstrated that the light-colored minerals K-feldspars mainly include microcline, bar-bierite and maximum microcline, nephelines including K-nepheline and cancrinite. All these rocks constitute an alkali-rich magmatic rock belt in space( about length 400 km), which can be divided into three subzones according to the features of petrochemistry from north to south: the northern subzone of alkaline syenite, middle subzone of alkaline granitoid and southern subzone of quartz syenite (porphyry). The three subzones of alkali-rich intrusive rocks belong to the same category of the high-K, ALK=10-15,w(K2O) = 5%-15%,w(K2O)/w(Na2O) = 1. 26-8. 30. The REE and trace element show: (1) All rocks have the same REE characters: The REE distribution pattern is a LREE-rich type without obvious Eu anomaly, the ratios of (wn(La)/wn(Yb) ranging between 10 and 30; (2)The rocks are enriched in LILE (Large Ion Lithosphile Element) with significant negative anomalies of Ba, Nb, Zr, indicating a character of continental rift related magmatic rocks; (3)The different alkali-rich intrusive rocks in the eastern Qinling were mostly derived from the sublithosphere, a small amount material was derived from the mantle and upperlithosphere. The authors have discussed the tectogenesis and the mechanism of alkali-rich magrnatic emplacement in this region on the basis of a detailed study on the Nd, Sr, Pb isotopic geochemistry of the alkali-rich intrusive rocks. This indicates that multiple splitting and multiple phases of alkaline magmatic activity have been taken place on the paleocontinental margin. Analysis of rock assemblages, structure and magma source and physico-chemical conditions shows that the alkali magma may come from crust and may have formed in a post-orogenic flabby and extensional structural environment.