浙西北木瓜燕山期花岗斑岩的定年、地球化学特征及其地质意义  

Age and geochemistry of the granitic porphyry from the northwestern Zhejiang Province,SE China,and its geological significance

作　　者：厉子龙[1] 周静[1] 毛建仁[2] 余明刚[2] 励音骐[1] 胡逸洲[1] 汪惠惠[1] 

机构地区：[1]浙江大学地球科学系,杭州310027 [2]中国地质调查局南京地调中心,南京210016

出　　处：《岩石学报》2013年第10期3607-3622,共16页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(2011CB808902);中国地调局南京地调中心项目(1212011121098);浙江省钱江人才计划(2010R10031);浙江省国土资源厅项目(2010001)联合资助

摘　　要：浙晥赣相邻区形成于晋宁期的木瓜岩体中发现有花岗斑岩和两期辉绿岩脉的出现。对其中花岗斑岩中锆石采用LA-ICP-MS技术测年,结果显示其侵位时间为142.2±1.2Ma,即形成于早白垩世。花岗斑岩具有高的SiO2,K2O+Na2O含量和K2O/Na2O比值,属过铝质的花岗岩。花岗斑岩和早期辉绿岩比晚期辉绿岩富集K、Rb等大离子亲石元素和Ta、Zr、Hf等高场强元素,但亏损Sr和Ti。花岗斑岩和两期辉绿岩的稀土元素配分模式均为LREE富集的右倾型,花岗斑岩中负Eu异常明显,早期辉绿岩次之,晚期辉绿岩无Eu异常,反映它们的斜长石结晶程度不同。在微量元素蛛网图中花岗斑岩和早期辉绿岩有较明显的Nb亏损,而晚期辉绿岩Nb亏损不明显。花岗斑岩的Y/Nb比值>1.2以及Nb,Ti和Sr亏损可能指示其来自壳源或具有来自与俯冲有关的物质。Sr-Nd-Pb同位素组成分析结果表明花岗斑岩的(87Sr/86Sr)i值为0.71134~0.71748,εNd(t)值为-3.3^-6.4,Pb同位素比值(206Pb/204Pb)i为18.49~18.56,(207Pb/204Pb)i为15.62~15.65,(208Pb/204Pb)i为38.57~38.68;早期辉绿岩的(87Sr/86Sr)i值为0.71408~0.73010,εNd(t)值为-0.05^+0.85。地球化学和同位素特征表明花岗斑岩为壳源成因,与早期辉绿岩脉和晚期辉绿岩脉均形成于陆内拉张环境中。而两期辉绿岩均来自地幔源区,且岩浆作用过程以部分熔融为主。通过对木瓜花岗斑岩与两期辉绿岩的研究并对比浙皖赣邻区的相关地质特征,笔者认为木瓜花岗斑岩和早期辉绿岩侵位于早白垩世早期华南岩石圈伸展背景下,随着拉张的持续,有广泛的晚期辉绿岩墙的侵位;结合浙皖赣地区123~129Ma时期出现的A型花岗岩也形成于陆内造山后伸展环境,指示了从145Ma以前的活动大陆边缘向145Ma以后陆内岩石圈减薄和伸展环境转变的一系列构造体制转换过程。结合前人的研究成果,认为浙西北及邻区从晚侏罗世至晚白垩世岩浆-构造演化过程Granites and diabases in the Mesozoic are two very important rock types in South China. We newly found granitic porphyry and two episodes of diabases within the Mugna Neoproterozoic gneissic granitic pluton in Chun＇ an area of the Northwestern Zhejiang Province （ZXB） of SE China. The age of 142. 2 ± 1.2Ma was obtained from the granitic porphyry by zircon U-Pb dating, belonging to the Early Cretaceous. Geochemically, the granitic porphyry is characterized by high SiO2, K2O ± Na20 contents and K2O/ Na2O, and being a peraluminous granite series. It was enriched Rb and light rare earth elements as well as high field strength elements （Ta, Ce, Zr, Hf） , and having moderate Nb depletion in the primitive mantle normalized diagram, indicating a source derived from subduction-related materials and/or crustal contamination. The early-stage diabase has similar chemical characters compared to the Mugua granitic porphyry. The late-stage diabase with no Eu anomalies has distinct composition compared to that of the early-stage diabase and granite porphyry with negative Eu anomalies. Granitic porphyry has Y/Nb ratios 〉 1.2, and depleted in Nb, Ti and Sr, indicating crustal origin with subduction zone signatures. Sr-Nd-Pb isotopic components show that the granitic porphyry has （^87Sr/ ^86Sr）; values of 0. 71134 ～ 0. 71748 and εNd （t） values of - 3.3 ～ - 6. 4, （^206pb/^204pb） i values of 18.49 ～ 18.56, （^207 pb/^204pb）i values of 15.62 - 15.65 and （^208 pb/^204 Pb ）i values of 38.57 -38. 68 ;and the early-stage diabase has （^87 Sr/^86 Sr）i values of 0. 71408 0～73010 and εNd （t） values of -0.05～ ＋ 0. 85. Based on the above study combined with the previous study, we suggest that the ZXB granitic porphyry may be derived from the Mesoproterozoic metamorphic rocks by partial melting induced by the heat of the deep mantle-derived hot magma upwelling. The diabases may be derived from mantle source. A geological comparison between ZXB and adjacent areas, we propose that the formation

关 键 词：地球化学特征 SR-ND-PB同位素 锆石U-Pb定年 侵入岩 岩浆演化 浙西北 

分 类 号：P588.13[天文地球—岩石学] P597.3[天文地球—地质学]