Microfossils from the siltstones and muddy slates:Constraint on the age of the Taowan Group in the Northern Qinling Orogenic Belt,Central China

The age of the Taowan Group is still a hot topic in the Northern Qinling Orogenic Belt, because of limited fossil evidence from these strata and different ideals on the Cambrian fossils of the conglomerate beds and limestone blocks. Early Ordovician acritarchs, chitinozoa and scolecodonts occur in the siltstone and muddy slate beds of the Guoling, Sanchakou, Fengmaimiao and Goushenmiao Formations of the Taowan Group, indicating the age of the Taowan Group should be the Ordovician. These fossil assemblages consist of 7 genera and 13 species of acritarchs, 7 genera and 9 species of chitinozoa, and 2 genera and 2 species of scolecodonts.

东秦岭陶湾岩群碳酸盐质糜棱岩的变质—变形特征及形成环境分析

东秦岭造山带北缘的陶湾岩群中分布着一套碳酸盐质糜棱岩,具明显的塑性变形,其展布和变质变形特征严格受洛南-栾川断裂带控制,是陶湾岩群碳酸盐岩在洛南-栾川断裂带晚加里东-早海西期构造活动过程中形成的.因此,对这套碳酸盐质糜棱岩的变质-变形特征及形成环境分析,可揭示洛南-栾川断裂带在这一时期构造活动的运动学、动力学及流变学特征.本文通过野外详细的观察和测量,结合室内显微构造分析和温压条件的计算,确定其变质相属低绿片岩相,变形机制是以方解石动态重结晶和细粒化作用为主的塑性变形.变形温度为350～420℃,差异应力为50 ～110 MPa.石英应变椭球体形态显示陶湾岩群遭受了强烈挤压变形.所以,陶湾岩群碳酸盐质糜棱岩是在强烈的斜向挤压碰撞环境下形成的,是秦岭微陆块向华北板块下斜向俯冲碰撞的结果.

东秦岭卢氏地区奥陶系陶湾群的变质变形特征及形成环境分析

陶湾群是一套经历多期变质—变形作用的碳酸盐岩、泥砂质岩及碎屑岩组合的地质体。它沿着洛南—栾川断裂带北侧呈长条状分布,其东西两侧的变形特征及产状完全不同,为了查明其变形特征及产状变化的因素,本文选择陶湾群中段卢氏地区的3条构造剖面为研究对象,通过系统考察陶湾群糜棱岩的宏观变形特征以及对面理、线理的测量和分析,认为陶湾群的塑性变形特征与其南侧洛南—栾川断裂带的特征一致,且具有近断裂带变形强,远离断裂带变形弱的特征;运动学特征表现为由南向北俯冲兼具左行走滑的特征。结合室内显微构造分析,认为陶湾群碳酸质糜棱岩的变形机制是以方解石亚颗粒式至高温边界迁移式动态重结晶为主的塑性变形,变形温度为170~250℃。其所受应力场方向有NE—SW向和NW—SE向两组,其中NE—SW向应力是早期宽坪群对陶湾群下俯冲而产生的挤压应力,NW—SE向应力是在区域上受太平洋板块的影响。石英应变椭球体形态显示陶湾群遭受的是挤压变形。洛南—栾川断裂带上(卢氏段)微弱糜棱岩化和未变形岩脉的锆石U-Pb同位素年龄为386 Ma,说明断裂带韧性变形活动结束于此时。由此,陶湾群碳酸质糜棱岩是在强烈的斜向挤压碰撞环境下形成的,是秦岭微陆块向华北板块下斜向俯冲碰撞的结果,其变质变形是洛南—栾川断裂带晚加里东—早海西期构造活动的响应。

豫西栾川地区“陶湾群”研究

本文探讨了栾川地区“陶湾群”的层序和时代,确定了在栾川牛栾—陶湾地区发育一大型韧性断裂带,所谓“秋木沟组”实际上是由韧性断裂带中的糜棱岩带和夹持其间的、由不同时代地层形成的中间岩块所组成,并确定了断裂带内中间岩块的地层归属。将原三岔口组解体为两部分,即震旦系三岔口组和下古生界陶湾组。所谓“陶湾群”实际上在不同地区包含了不同时代和不同岩性的地层,不是一个可以在一定空间进行对比的地层单元,因而作为一个“群”是不合适的。

陕西陶湾群北侧剪切混杂带的确定及其地质意义

陕西境内的陶湾群和大片出露的寒武纪地层之间,发育一套以千枚岩夹白云岩透镜体为主的复杂岩系。这套岩系实际上构成一个剪切混杂带,成为秦岭造山带和华北地台的界线。该带不是一个沉积地层单位,而应视为岩石-构造组合体,带内发育一系列冲断层。在洛南柏峪寺地区,从中可以分出3个构造岩片,其中的白云岩岩块为构造岩块。根据显微、小构造分析及岩性对比,认为冲断层由北向南逆冲,带内至少卷入有晚元古代大庄组、震旦纪罗圈组、中下寒武统及少量陶湾群。后期的强烈的构造变动使原地层支离破碎,层序发生了很大变化。

陶湾群层序及时代研究

陶湾群分布于洛南—栾川一带,分三个自然段出露。本文描述了各地段陶湾群代表性剖面,论述了层序划分和区域对比。文中依据地层接触关系、古生物资料以及同位素地质年龄,确定陶湾群的时代为奥陶纪。

Tectonic Boundary and Ceasing Time of Amalgamation between the North China Craton and the North Qinling Belt

The Qinling orogenic belt is a composite orogen, consisting of the north and south Qinling belts with different collision ages. The North Qinling belt is composed of several petro-tectonic units, including the Kuanping, Erlangping, Qinling and Danfeng groups with numerous granitoid plutons from Neoproterzoic to Middle Devonian. To the north of the Kuanping Group, a mylonitized granite body in the Luanchuan Group was determined to have the magmatic age of 850 Ma, which is the northernmost Neoprotozoic granite body. The volcanic rocks of the Dahongkou Formation from the Luanchuan Group had the eruptive age of -1 600 Ma, obtained from zircon U-Pb LA-ICP-MS analysis. The detrital zircons in meta-sandstones from the Taowan Group have three populations of-920-1 400, -1 600-2 100, and 〉2 200 Ma ages, whereas those from the Kuanping Group shows six populations with the peaks of -590, -767, -952, -1 590, -2 485, and -3 200 Ma. These data suggest that the boundary between the North China Craton and the North Qinling belt cannot simply be constrained by using the events determined from zircon U-Pb ages. The granitoid plutons with the magmatic age of 430 Ma were strongly deformed, while those with the magmatic age of -390 Ma show no or less deformation, indicating that the deformation of amalgamation between the North China Craton and the North Qinling belt ceased between 390 and 430 Ma.