摘要
浙江淳安木瓜地区基性岩墙群以NW-SE向产出,岩性主要为辉绿岩和辉绿玢岩,地球化学分析显示其主要属于拉斑玄武岩系列。稀土元素分布模式均表现为右倾型,(La/Yb)N=5.1～7.6,平均为6.4,反映轻重稀土元素分异较为明显,重稀土元素明显亏损,表明源区有石榴子石的残余。La/Sm-La图解显示本区基性岩墙的岩浆是平衡部分熔融形成;δEu为0.95～1.03,平均为0.99,Eu异常不明显,暗示无斜长石的分离结晶作用。本区基性岩墙相对于洋中脊玄武岩明显富集大离子亲石元素K、Rb、Cs、Ba、Sr和高场强元素Nb、Ta、Zr、Hf、Ti,相容元素Cr、Ni、Co与MgO的正相关性指示原始岩浆经历了一定程度的分离结晶作用。本区基性岩墙的LA-ICPMS锆石U-Pb加权平均年龄为(135±2)Ma,这表明其是继华南印支期陆内挤压变形之后,早白垩世岩石圈发生伸展减压作用所诱发的地幔物质部分熔融的产物,反映早白垩世扬子地块东南缘处于强烈的裂解和岩石圈伸展减薄的构造环境与动力学背景。
The mafic dyke swarms in Mugua-Chun＇an area, Zhejiang Province mainly consist of diabase and diabase porphyrite with NW-SE trend. The rocks are mainly tholeiitic according to geochemical analyses. The REE distribution patterns show strong LREE-enrichment, with （La/Yb）N ratios of 5.1 -7.6 （mean 6.36）, indicating a strong fractionation between LREE and HREE and possible residual garnet in the source region. The La/Sm vs. La diagram shows that the mafic dyke swarms in this region are mainly the production of equilibrium partial melting. The Eu anomaly is not obvious （δEu 0.95 1.03, with an average value of 0.99）, reflecting that there was no prior plagioclase separation from the source region. Furthermore, the mafic dyke swarms are enriched in LILE （K, Rb, Cs, Ba, Sr） and HFSE （Nb, Ta, Zr, Hf, Ti） compared to mid-ocean ridge basalt. The positive relationship between the compatible elements （Cr, Ni, Co） and Mg^2＋ indicates that the original magma has undergone fractional crystallization to a certain degree. The LA-ICPMS U-Pb dating of zircons from the mafic dyke swarms yielded a weighted mean age of （135±2） Ma, suggesting these rocks are the products of partial melting of mantle material caused by the lithospheric extension and decompression during the Early Cretaceous after the South China Indosinian intra-continental compressional deformation. This may reflect the southeast margin of the Yangtze Block was in the tectonic environment and geodynamic setting of intense break-up and lithospheric extensional thinning during the Early Cretaceous.
出处
《地球化学》
CAS
CSCD
北大核心
2011年第1期22-34,共13页
Geochimica
基金
浙江省国土资源厅项目(2007002)
国土资源部公益性行业科研专项经费项目(200811015)
