北祁连川刺沟A型花岗岩的年代学及其意义

Geochronology of the Chuancigou A-type granite in the North Qilian belt and its significances

关于北祁连南蛇绿岩带蛇绿岩,前人多认为是形成于早古生代洋中脊的大洋岩石圈残片。本次对川刺沟A型花岗岩和其周围超基性岩的研究,有助于较全面认识该区时代序列与构造格架。川刺沟A型花岗岩的组成矿物为碱性长石(～45%)、石英(～30%)、斜长石(～15%)以及少量的霓辉石(～5%)等,发育条纹和显微文象等结构。岩石具有高SiO_2(72.58%～73.96%)和高Na_2O+K_2O(10.12%～10.39%),A/NK<1,10000Ga/Al值为2.59～3.49。微量元素型式显示该花岗岩富集Rb、Ba、Th、K,强烈亏损Sr、P、Ti、Nb、Ta,稀土元素型式呈明显负铕异常(δEu=0.56～0.60)的海鸥形。岩相学和地球化学特点均符合A型花岗岩特征,进一步细分可将其归属于A_2型,形成于后造山环境。其周围的超基性岩稀土型式呈"U"型,富集Th、Zr、Hf,推测其经历过俯冲熔/流体交代。SHRIMP锆石U-Pb定年给出一组该A型花岗岩的谐和年龄501±5Ma,可以代表其侵位年龄。该年龄早于北祁连造山带陆-陆碰撞时间,晚于大陆裂谷发生时间。由于A_2型花岗岩后造山环境有两亚类:陆-陆碰撞后造山和活动大陆边缘拉张环境,本次研究结果更加倾向后者,即,该A型花岗岩为在中-晚寒武世俯冲作用影响下,活动大陆边缘弧前发生拉张,软流圈部分熔融产生幔源岩浆底侵形成。进一步推测该地区超基性岩可能与俯冲有关,为非经典定义的蛇绿岩。

Some geologists believe that South Ophiolite Belt in North Qilian is a fragment of oceanic lithosphere formed in ocean ridge in Early Paleozoic. Characteristics of an A-type granite and a peridotite in the belt will help to understand the time sequence and structural framework of this area. This A-type granite is mainly composed of alkali feldspar （-45%）, quartz （-30%）, plagioclase （-15%）, and aegirine augite （-5%） and other accessory minerals as well, with perthitic and micrographic textures. The granite is high in SiO2 （72.58%-73.96%）, Na2O＋K2O （10.12%-10.39%） and Ga/Al （10000Ga/Al=2.59-3.49）. Its A/NK ratios are below 1.0, which means the granite is peralkaline. Normalized rare earth element and trace element patterns show that this pluton has seagull-shaped REE patterns with obvious negative Eu anomaly （δEu=0.56-0.60）, enriched in Rb, Ba, Th, K and depleted in Sr, P, Ti, Nb, Ta. Petrological and geochemical characteristics suggest that this granite is an A-type one, which is subdivided into A2-type, i.e., formed in post-orogenic environment. U-shaped REE patterns and the enrichment of Th, Zr, Hf of the Chuancigou peridotite illustrate a source of fossil mantle wedge modified by melts or fluids released from subduction slab. SHRIMP U-Pb analysis on zircon of the granite yields a weighted mean age of 501±5Ma, interpreted as the emplacement age, which is earlier than the event of continent-continent collision and later than continent rifting of the North Qilian orogenic belt. As there are two sub-types for A2 granite：post-collision and active continental margin, for the Chuancigou A2 granite, the forming environment should be active continental margin. In the Middle-Late Cambrian fore-arc of active continental margin of North Qilian, it is inferred that asthenosphere melted due to the extension caused by subduction and the magma underplated to form the Chuancigou A-type granite, and the formation of the ultramafic rock in the studied area is estimated to be related to subduction zone and it is not a classically defined ophiolite.