松潘-甘孜地体青海吾和玛高分异Ⅰ型花岗岩的识别与岩石成因

Identification and petrogenesis of Wuhema highly fractionated Ⅰ-type granitoids in Qinghai Province from Songpan-Ganzi Terrane

为探讨松潘-甘孜地体内青海吾和玛花岗质岩石的成因类型与岩石成因,对其开展了岩相学、地球化学、锆石U-Pb年代学和Lu-Hf同位素等研究。吾和玛花岗质岩石被确定为高分异的Ⅰ型花岗岩,其结晶年龄为216.6±4.3 Ma,为正长花岗岩,具有弱过铝质高钾钙碱性特征。岩石轻、重稀土元素分馏不明显((La/Yb)_(N)<7),轻稀土元素轻微富集,重稀土元素分布平缓,具有明显的负Eu异常(δEu=0.08~0.43),明显亏损Ba、Sr和高场强元素Nb、Ta、P、Ti,富集大离子亲石元素Rb、Th、U、K,ε_(Hf)(t)值为-2.54~1.32。吾和玛花岗质岩石的母岩浆是岩石圈拆沉作用引起软流圈上涌,诱发中元古代地壳物质(类似于变杂砂岩的物质)部分熔融形成长英质母岩浆,后期又受到斜长石、钾长石、磷灰石等矿物分离结晶作用的控制,经历了高程度的分异形成的。

In order to study the genetic type and petrogenesis of the Wuhema granitoids in the Songpan Ganzi Terrane,the petrography,geochemistry,zircon U-Pb chronology and Lu-Hf isotope have been studied in this paper.The Wuhema granitic rocks are identified as highly fractionated Ⅰ-type granitoids with crystallization age of 216.6±4.3 Ma.The rocks are syenogranite,and have the characteristics of weakly peraluminous,high-K calc-alkaline.The fractionation of REE in the rocks is not obvious(La/Yb)_(N)<7,light rare earth elements are slightly enriched,and heavy rare earth elements are gently distributed.The rocks have obvious negative Eu anomalies(δEu=0.08~0.43),with obvious depletion of Ba,Sr and high field strength elements(HFSEs,e.g.,Nb,Ta,P and Ti),and enrichment of large ion lithophile elements(LILEs,e.g.,Rb,Th and U),ε_(Hf)(t)values of the rocks range from-2.54 to 1.32.The primary magma of the Wuhema granitic rocks were formed by the upwelling of asthenosphere caused by the lithosphere delamination,and induced the partial melting of Mesoproterozoic crustal materials(similar to metagreywacke),then the magma went through a high degree of fractionation of plagioclase,potassium feldspar,apatite and other minerals.