中国北天山晚石炭世白杨沟火山岩的成因及地质意义

Petrogenesis and geological implications of the Late Carboniferous Baiyanggou volcanic rocks

中国北天山作为中亚造山带西部重要的组成部分,其晚古生代的构造背景长期存在板内裂谷环境和岛弧环境两个截然不同的认识,有些学者还提出该地区有塔里木地幔柱的影响,从石炭纪到二叠纪均发育大量的双峰式火山岩。为了厘定其石炭-二叠纪模糊不清的构造属性以及火山岩的地质特征,本文对北天山博格达隆起带白杨沟地区火山岩进行了系统的研究。这套火山岩由枕状玄武岩、块状玄武岩、安山-英安质熔结凝灰岩、流纹岩和火山角砾岩组成。安山-英安质熔结凝灰岩和流纹岩属于Ⅰ型酸性岩,与枕状玄武岩及块状玄武岩整合接触。海相棘皮类化石的发现以及锆石SHRIMP U-Pb年龄(~311Ma)的测定指示这套白杨沟火山岩应属于晚石炭世祁家沟组。同时,安山-英安质熔结凝灰岩的发现表明白杨沟火山岩剖面并非双峰式火山岩,但与双峰式岩浆(玄武质和流纹质岩浆)有密切的成因关系。MELTS模拟计算指示白杨沟流纹岩和熔结凝灰岩不是与其共生的玄武岩高度结晶分异的产物。与含水玄武质岩石的部分熔融实验对比,正的ε_(Nd)(t)值(+5.9^+7.5)以及岛弧特征的微量元素性质则表明白杨沟流纹岩和熔结凝灰岩主要由含水的新生岛弧玄武质地壳(岩石)发生部分熔融形成。流纹岩很可能代表新生地壳部分熔融的直接产物。然而熔结凝灰岩中发育玄武质和长英质两类浆屑,大量斜长石晶屑的发育以及负Eu至正Eu异常(δEu=0.8~1.1)暗示安山-英安质熔结凝灰岩还受岩浆混合作用和长石堆晶作用的影响。结合博格达晚石炭世玄武岩的研究,本文认为博格达晚石炭世应为洋内岛弧后弧或弧后环境,与北天山洋(或称为准噶尔洋)向南俯冲有关。博格达隆起带石炭-二叠纪构造属性的转变很可能与东准噶尔弧和博格达弧在石炭-二叠纪界限时期发生的弧-弧碰撞作用有关。

Chinese Northern Tianshan Orogen is one of the important components of the Central Asian Orogenic Belt （CAOB） as its western part. Considerable debates persist as to its tectonic setting during the Late Paleozoic era, among which two dominant schools of quite different opinions were presented, one thought it was formed in an active subduction system while the other in an intraplate rift environment. Some researchers ever suggest that lots of bimodal volcanics occur in Chinese Northern Tianshan from Carboniferous to Permian, which are related to the Tarim mantle plume. To provide new constraints on this issue, petrology and geochemical analyses have been carried out on the Baiyanggou volcanics from the Bogda Mountains. The Baiyanggou volcanics are composed of pillow basalt, massive basalt, andesitic-dacitic ignimbrite, rhyolite and volcanic breccia. Andesitic-dacitic ignimbrite and rhyolite belong to I-type felsic rock and are in conformable contact with the pillow and massive basalts. Marine Echinodermata fossil and zircon SHRIMP U-Pb age （～311Ma） indicate that these volcanics are formed in Late Carboniferous （the Qijiagou Formation）. Furthermore, occurrence of andesitic-dacitic ignimbrite suggests that the eruption of the Baiyanggou volcanics is not a bimodal volcanism, but related to bimodal magma （basaltic and rhyolitic magmas）. MELTS modeling implies that the felsic volcanics are not produced by fractional crystallization of the pillow or massive basalt. Comparison with previous basaltic melting experiments, positive εNd（t） values （＋5.9～＋7.5） and arc-like trace element characteristics indicate that they are produced by partial melting of neonatal hydrated arc mafic crustal rocks. Rhyolites likely represent the direct product of neonatal arc crustal rocks. Nevertheless, ignimbrites contain lots of basaltic and silicic fiammes and feldspars and display negative to positive Eu anomalies （δEu=0.8～1.1）, implying that magma mingling and feldspar accumulation may have played an important role in the genesis of the ignimbrites. Combined with the Bogda Late Carboniferous basalts, we suggest that it may be an intra-oceanic rear-arc or back-arc environment in the Bogda belt during Late Carboniferous, probably related to southward subduction of the North Tianshan Ocean （also named as Junggar Ocean）. The transition of the tectonic settings of the Bogda arc is likely related to a ＂soft＂ arc-arc collision （East Junggar arc vs. Bogda arc） at the boundary of Carboniferous-Permian.