摘要
班公湖-怒江结合带东段是破译班公湖-怒江特提斯洋早期演化的重要窗口。在对八宿县怒江大桥-邦达地区嘉玉桥岩群详细野外观察基础上,对赋存其中的玄武岩岩块及其围岩进行了锆石U-Pb定年,并对玄武岩进行了全岩地球化学研究。嘉玉桥岩群具有构造混杂岩的特征,基质主要为板岩、千枚状板岩、薄层状结晶灰岩,岩块主要为玄武岩、结晶灰岩、大理岩。玄武岩岩块锆石的外部形态、内部结构及均一的年龄结果显示其为基性岩浆结晶锆石。基性锆石^(206)Pb/^(238)U年龄的加权平均值为338±2Ma,代表玄武岩形成时代。玄武岩的围岩中碎屑锆石U-Pb年龄范围为3157~500Ma,最年轻一组年龄为513~500Ma,限定其原始沉积时代不早于该年龄范围。玄武岩轻稀土元素亏损,高场强元素Nb、Ta、Zr、Hf无异常,ε_(Nd)(t)值为+7.4~+8.5,这些地球化学特征与N-MORB型玄武岩一致。受海水蚀变影响,Rb、Ba、U、Sr元素和^(87)Sr/^(86)Sr(i)同位素有不同程度的富集。玄武岩岩块是由亏损地幔中尖晶石相二辉橄榄岩部分熔融形成,代表洋壳残片。八宿县怒江大桥-邦达地区嘉玉桥岩群是一套俯冲增生杂岩,其基质岩石和岩块主要形成于早石炭世,构造就位时间有待进一步限定。班公湖-怒江结合带存在较多前侏罗纪的化石和同位素年龄信息,本研究揭示班公湖-怒江洋在338±2Ma已经具有成熟的洋壳,指示班公湖-怒江特提斯洋打开时间应早于早石炭世(338±2Ma)。
The eastern segment of the Bangonghu-Nujiang suture belt is the key area to decipher the early evolution of the Bangonghu-Nujiang Tethys Ocean.On the basis of detailed field observation of the Jiayuqiao Group in Nujiang River bridge-Bamda area,Baxoi,the zircon U-Pb dating of the basaltic blocks and their surrounding rock were carried out,and the whole-rock geochemistry of the basalts were studied.The Jiayuqiao Group is characterized by tectonic mélange,and the matrix rocks are mainly slate,phyllitic slate and thin-bedded crystalline limestone,and the blocks are mainly basalt,crystalline limestone and marble.The external morphology,internal structure and uniform age of the zircons extracted from the basalt indicate that they are magmatic crystalline zircons.The weighted average age of ^(206)Pb/^(238)U is 338±2Ma,which represents the age of basalt formation.The detrital zircons U-Pb age of the surrounding rocks ranges from 3157Ma to 500Ma,and the youngest group ranges from 513Ma to 500Ma,which limits the original sedimentary age of the surrounding rocks not earlier than this age range.The light rare earth elements(LREE)of basalts are depleted in chondrite-normalized REE pattern,and the high field strength elements(Nb,Ta,Zr,Hf)are no anomalies in the primitive mantle-normalized trace element pattern.Theε_(Nd)(t)value ranges from+7.4 to+8.5.These geochemical characteristics are consistent with the normal mid-ocean ridge(N-MOR)type basalt.Rb,Ba,U,Sr and ^(87)Sr/^(86)Sr(i)isotopes are enriched in different degrees due to seawater alteration.The basalts are formed by partial melting of spinel peridotite in depleted mantle and represent oceanic crust fragments.The Jiayuqiao Group in Nujiang River bridge-Bangda area is a set of subduction accretionary complex.Its matrix rocks and tectonic blocks were mainly formed in the Early Carboniferous,and the tectonic emplacement time needs to be further defined.There are many pre-Jurassic fossil and isotopic age information in the Bangonghu-Nujiang suture.This study reveals that the Bangonghu-Nujiang Tethys Ocean had mature oceanic crust at 338±2Ma,suggesting that the opening time of the Bangonghu-Nujiang Tethys Ocean should be earlier than the Early Carboniferous(338±2Ma).
作者
王冬兵
王保弟
潘桂棠
罗亮
唐渊
WANG DongBing;WANG BaoDi;PAN GuiTang;LUO Liang;TANG Yuan(Chengdu Center,China Geological Survey,Chengdu 610081,China;China Aero Geophysical Survey and Remote Sensing Center for Natural Resources,Beijing 100083,China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2021年第10期3048-3066,共19页
Acta Petrologica Sinica
基金
第二次青藏高原综合科学考察研究项目(2019QZKK0702)
中国地质调查局项目(DD20190053)
国家自然科学基金项目(41802031)联合资助.
关键词
班公湖-怒江结合带
嘉玉桥岩群
洋壳残片
俯冲增生杂岩
特提斯
青藏高原
Bangonghu-Nujiang suture belt
Jiayuqiao Group
Remnants of oceanic crust
Subduction accretionary complex
Tethys
Tibetan Plateau
