摘要
湖南阳明山复式岩体位于扬子地块和华夏地块的结合部位,岩体主要由白果市-土坳黑(白)云母二长花岗岩、阳明山二云母二长花岗岩、大江背电气石白云母二长花岗岩、大源里黑云母正长花岗岩组成,利用LA-ICPMS锆石U-Pb定年分别给出228.6±1.4Ma和205±1.8Ma、229.0±2.0Ma和221.8±1.3Ma、218.2±2.0Ma、217.8±1.6Ma的谐和年龄。综合野外观察、地质测量,阳明山复式花岗岩体可能经历了4期岩浆活动:~229Ma、~221Ma、~217Ma、~205Ma。各期次岩石总体具有高碱(ALK平均值8.55%)、高铝(Al2O3平均14.47%)、高磷(P2O5平均值0.25%),低硅(SiO2平均值71.74%)、低铁(Fe2O3T平均值0.92%)、低镁(MgO平均值0.19%)低Ca(CaO平均值0.64%)等特征。铝饱和指数A/CNK值为0.97%~1.30%,大部分大于1,属高钾钙碱性过铝-强过铝花岗岩。稀土配分模式表现为轻稀土相对富集,重稀土相对亏损,具有强烈的负Eu异常。在标准化蛛网图上,富集大离子亲石元素(U、Th、K)和Pb,明显亏损高场强元素(Ti、Nb)和Ba、Sr等元素。上述特征指示阳明山花岗岩是印支晚期同源岩浆多次脉动侵入的复式岩体并具有S型花岗岩的地球化学特征。锆石的εHf(t)值均为负值(集中在-12^-5之间),两阶段模式年龄(tDM2)主要集中在1.28~1.60Ga,推断是古老地壳部分熔融的产物。结合其形成年龄和区域构造背景,本文认为阳明山复式岩体是印支运动由挤压构造向伸展构造转换加厚地壳发生局部伸展-减薄的机制下,中元古代变质泥岩部分熔融形成,并经历了分离结晶作用,其形成温度850~875℃,压力约为7×108~10×108Pa,相当深度为26~30km。
The Yangmingshan granite intrusions from Hunan are located at the junction area between the Yangtze block and Cathaysia block. They consist of Baiguoshi-Tuao biotite （muscovite） monzonitic granites, Yangmingshan two-mica monzonitic granites, Dajiangbei tourmaline muscovite monzonitic granites, Dayuanli biotite syenogranite granites, and LA-ICPMS zircon U-Pb dating show that 228. 6 ± 1.4Ma and 205±1.8Ma, 229. 0 ± 2. 0Ma and 221.8± 1.3Ma, 218.2 ±2. 0Ma, 217. 8± 1.6Ma, respectively. In conjuction with our field invest tigations, Yangmingshan granite inferred that there may be four-episode magma activities： -229Ma, - 221Ma, - 217Ma, - 205Ma. The granites are characterized by high alkali （average 8.55% ） , Al2O3 （ average 14. 47% ） and P2O5 （average 0. 25% ） and low SiO2 （ average 71.74% ）, TFe203 （ average 0. 92% ）, MgO （ average 0. 19% ）. A/CNK varies from 0. 97% to 1.30%, mostly 〉 1, belongs to high-K calc-alkaline and peraluminous series. They have LREE enrichments relative to HREE, and evidently Eu negative anomalies. In the spider diagram,they show enrichments in LILE （U, Th, K and Pb） and marked depletions in HFSE （Ti and Nb）, Ba and Sr. These observations are characteristics of S-type granite and were regarded as a complex rock-mass resulted from repeated pulsating intrusion of magma from the same origin. Their zircons have negative εHf（t） values （ -12 - - 5） and two-stages model ages （ 1.28 - 1.60Ga）, suggesting that the Yangmingshan granites were formed by partial melting of old crust. Combined zircon U-Pb dating and regional tectonic setting, we think that the Yangmingshan intrusions were formed by the decompression melting of Mesoproterozoic metamorphic mudstone in the environment of partial extension-thinning of the thicken lower crust, and the magma then suffered fractional, crystallization, when the Indosinian movement transformed from squeezing tectonic to extension tectonic. Their formation temperatures are 850 - 875-C and pressures are 7 ×10
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2014年第5期1485-1504,共20页
Acta Petrologica Sinica
基金
中国地质调查局项目(1212011220522
K201134
12120113061700)资助
