北秦岭西段尹道寺中生代火山岩及兴隆山新元古代火山岩岩石地球

1、北秦岭西段尹道寺中生代火山岩及兴隆山新元古代火山岩岩石地球        【中文摘要】本文在充分搜集研究前人资料的基础上,结合野外实际路线调查及重要样品实验测试研究,以地球化学研究为主,通过区域地质学、构造地质学、岩石大地构造学、同位素年代学及岩相学等方面的分析重点对北秦岭西段伯阳-尹道寺地区及祁连东段兴隆山地区火山岩系进行系统研究,确定其形成时代,识别其成因和大地构造环境,阐明岩石成因和构造演化历史,为探讨中国大陆造山带的基本特征及大陆动力学机制提供重要基础资料。通过对以上研究,取得以下主要新进展和初步熟悉:(一

2、)北秦岭西段尹道寺火山岩形成时代及岩石学研究结论LA-ICPMS锆石U-Pb定年以及元素地球化学研究表明,北秦岭西段尹道寺流纹岩形成于211±2.6Ma,为印支末期。总体说来,该套流纹岩具有如下元素地球化学特征:(1)SiO_2、Na_2O+K_2O含量高,TiO_2含量低,属亚碱性系列;(2)轻稀土高度富集,Eu负异常明显,具有大陆板内火山岩的微量元素原始地幔标准化分配模式,Ba、Sr、P、Ti呈明显亏损;(3)Pb同位素特征显示流纹岩的源区为北秦岭基底岩石,为下地壳部分熔融的产物,稀土元素和微量元素分配模式对比表明源区物质可能为古元古代秦岭岩群中的黑云角闪斜长片麻岩;残留物可能主

3、体为斜长石,含有角闪石,岩浆演化过程中存在程度有限的碱性长石和黑云母的分离结晶;(4)印支末期特定的构造转换背景与北秦岭西段尹道寺地区特定的断裂构造的结合,形成了该地区独特的流纹岩浆喷发。(二)祁连东段兴隆山群基性火山岩的时代及岩石学研究结论:(1)兴隆山群可分为下、中、上三个组,主体为一套浅变质碎屑岩与基性火山岩建造组合。上组基性火山岩样品的锆石U-Pb LA-ICPMS测年显示其形成年龄为713- 824Ma,表明兴隆山群上组形成时代为新元古代。(2)元素地球化学研究揭示兴隆山群下组玄武岩和中、上组的基性熔岩具有明显的差异,前者LREE富集,微量元素原始地幔标准化分配型式明显呈现Nb-Ta

4、、Ti负异常;后者LREE亏损,重稀土相对富集,微量元素原始地幔标准化分配型式呈平坦型。兴隆山群火山岩均为亚碱性拉斑系列火山岩。(3)兴隆山群基性火山岩源岩浆演化过程中经历了plag+cpx±ol的分离结晶作用。下组玄武岩形成于大陆板内伸展环境,源岩浆在上升演化过程中经历了较为强烈的地壳混染作用。中、上组形成于初始洋盆环境。(4)下组玄武岩幔源性质显示为EN(富集组分),反映了软流圈似地幔柱源上升过程中经受了地壳混染作用,主要形成于原始石榴子石稳定区域;中、上组基性熔岩幔源性质为亏损地幔源,类似于N-MORB,形成于亏损尖晶石二辉橄榄岩稳定区域。(5)兴隆山群是Rodinia超大陆形

5、成后从大陆伸展-大陆裂谷发育-初始洋盆形成的演化历史在祁连造山带东部的响应。');【Abstract】 Based on the former data, field investigation and major samples analysis and study, this * emphasizes particularly on geochemical and tectonic evolution of Mesozoic volcanic rocks from Yindaosi area in west segment of North Qinling Mts., and La

6、te Proterozoic volcanic rocks from Xinglongshan Formation in Eastern Qilian Mts., by means of geochemistry, regional geology, tectonic geology, petromorphology, isotope chronology and microphysiography. The purpose is to determine their age, identify their petrogenesis and geotectonic setting, and c

7、larify the relationship between the petrogenesis and tectonic environment. To study on the two groups volcanic rocks provided us important information for discussing the basic characteristics of Chinese continental orogenic zone and continental dynamics mechanism.Through the study, new findings are

8、as following:1. The results of U-Pb zircon La-ICPMS dating for volcanic rocks from Yindaosi area in west segment of North Qinling Mts. and the petrography study are as follow:The results of U-Pb zircon La-ICPMS dating for volcanic rocks from Yindaosi area in west segment of North Qinling Mts. and th

9、e elements geochemistry study show that Yindaosi rhyolite is 211±2.6Ma, at the later Indo-Chinese epoch.As a whole, the element geochemistry characteristics of this rhyolite are following.(1) High SiO_2, Na_2O + K_2O and low TiO_2, and it belongs to sub-alkali series.(2) LREE enriched, distinct

10、 negative abnormity on Eu, and the primitive-mantle normalized trace element patterns is of the intraplate, with the markedly depletion on Ba, Sr, P and Ti.(3) The traits of Pb isotope show that the source region of Yindaosi rhyolite is the basement rocks in the North Qinling Mts., which are produce

11、d by the partial melting earths crust. The comparison of Chondrite-normalized patterns of REE and the primitive-mantle normalized trace element patterns suggests that the matter in the source region may be biotite- hornblende-anorthose gneiss of Paleoagnotozoic Qinling Group. The residue may be main

12、ly plagioclase, containing hornblende, and the limited separate crystal of alkalic feldspar and biotite existed in the magma process.(4) Combining the specific conversion tectonic setting in Indo-Chinese epoch with certain faults in the Yindaosi area in west of North Qinling Mts., the rhyolite magma

13、 erupted particularly in the region.2. The results of U-Pb zircon La-ICPMS dating for volcanic rocks from Xinglongshan Formation in Eastern Qilian Mts. and the petrography study are as follow:(1) Xinglongshan Group consisting mainly of detrital rocks and basic volcanic rocks, which can be divided in

14、to lower, middle and upper formations. The result of U-Pb zircon LA-ICPMS dating for upper formation basic lava is 713-824Ma, together with the regional geological characteristics, we suggest that Neoproterozoic is the formation age of Xinglongshan Group.(2) Based on petrographic, major and trace el

15、ement data, the lower formation of basalt or basaltic and middle and upper formations of basic volcanic rocks of Xinglongshan Group are different distinctly. The former exhibits LREE enriched, distinct Nb-Ta ant Ti depletion in trace element patterns. The latter with LREE depleted and HREE enriched,

16、 their primitive-mantle normalized diagram show a“flat-type”patterns. The Xinglongshan Group volcanic rocks are dominantly tholeiitic in composition.(3) The parent magma of Xinglongshan Group basic volcanic rocks subjected to shallow level gabbroic (palg+cpx±ol) fractionation. The basic lavas i

17、n the lower formation occurs within plate rift setting, and crustal contamination has also contributed significantly to the formation of the basalt. In contrast, the middle and upper formation form in initial oceanic basin setting(4) The basalt or basaltic in the lower formation with En(composition enriched), exhibits approximate mantle plume in asthenosphere subjects crustal contamination in the process of uplifting , mainly forming in the primitive garnet stability field. The basic lavas in the middle, upper formation derived from depleted mantle source similar to N-MORB in inspinel