湘南都庞岭复式花岗岩成因及地质意义:矿物化学、锆石UPb年代学、地球化学与Nd-Hf同位素制约

Petrogenesis and implications of the Dupangling compound granite in southern Hunan Province,China:Constraints from mineralogical chemistry,zircon U-Pb age,geochemistry and Nd-Hf isotope

文章对位于南岭西段湘桂交界处的都庞岭东侧岩体开展了锆石SHRIMP U-Pb年代学、岩石学、矿物化学、岩石地球化学和Sm-Nd、Lu-Hf同位素分析研究。锆石SHRIMP U-Pb定年结果显示,粗中粒斑状黑云母二长花岗岩年龄为215.6±2.1 Ma,中粒斑状黑云母二长花岗岩年龄为220.5±1.8 Ma,中粒环斑黑云母二长花岗岩年龄为222.8±1.5 Ma,结合以往研究获得的细粒白云母二长花岗岩年龄209.7±3.1 Ma,认为岩体侵位时限介于222.8~209.7 Ma,为印支期岩浆活动产物,非以往认为的燕山期。环斑钾长石、黑云母聚晶的矿物化学特征表明环斑黑云母二长花岗岩形成过程中岩浆温度、压力、成分发生震荡变化,在玄武质岩浆的底侵作用下发生多次熔融作用形成黑云母聚晶。都庞岭黑云母二长花岗岩具有较高的SiO_(2)和K_(2)O+Na_(2)O含量,A/CNK值为1.02~1.39,里特曼指数(δ)为0.93~2.18,属过铝质钙碱性系列;微量元素地球化学性质表现为富集REE、Rb、Th和U及较高的HFSE(Nb、Y和Ga),亏损Ba、Sr、Eu,具有高的TFeO/MgO、Ga/Al比值,地球化学特征显示为A型花岗岩;Nd同位素ε_(Nd)(t)值为-8.74~-8.13,T_(2DM)值为1.71~1.66 Ga;锆石Hf同位素ε_(Hf)(t)值为-14.1~-1.4,T_(2DM)值为2.14~1.34 Ga,显示都庞岭黑云母二长花岗岩主要源于古老地壳物质的部分熔融,并受到了一定程度的亏损地幔物质的混染。印支运动的变质峰期在258~243 Ma,233 Ma以后华南地区处于伸展的构造背景并受到幔源玄武质岩浆大范围底侵,诱发地壳物质重熔形成伸展背景下的都庞岭印支期铝质A型(环斑)花岗岩。

In this paper,zircon SHRIMP U-Pb geochronology,petrology,mineral chemistry,petrogeochemistry,Sm-Nd and Lu-Hf isotopes were studied for the eastern part of the Dupangling pluton,which is located at the western section of Nanling at the junction of Hunan and Guangxi provinces.The zircon SHRIMP U-Pb dating results show that the age of coarse-to-medium-grained porphyritic biotite monzogranite is 215.6±2.1 Ma,mediumgrained porphyritic biotite monzogranite 220.5±1.8 Ma,medium-grained rapakivi biotite monzogranite 222.8±1.5 Ma.Combined with the age of fine-grained muscovite monzogranite as 209.7±3.1 Ma obtained in the previous research,it is suggested that the emplacement time ranges from 222.8 Ma to 209.7 Ma,and the pluton is derived from the Indosinian magmatic activity rather than the Yanshanian as previously thought.The mineral-chemical characteristics of rapakivi K-feldspar and biotite phenocryst indicate that the magma temperature,pressure and composition have gone through fluctuation during the formation process,and biotite phenocryst was formed by multiple melting under the underplating by basaltic magma in this area.The Dupangling biotite monzogranites have higher contents of SiO_(2) and K_(2)O+Na_(2)O,A/CNK value ranges between 1.02 and 1.39 and Rittman index(δ)between 0.93 and 2.18,belonging to the peraluminous calc-alkaline.Moreover,these monzogranites are enriched in REE,Rb,Th and U with higher content of HFSE(Nb,Y and Ga),depleted in Ba,Sr and Eu with higher ratios of TFeO/MgO and Ga/Al,showing the characterisitics of A-type granite.The isotopeε_(Nd)(t)value ranges from-8.74 to-8.13,and the T_(2DM) value from 1.71 to 1.66 Ga;the zircon isotopeε_(Hf)(t)value ranges from-14.1 to-1.4,and the T_(2DM) value from 2.14 to 1.34 Ga.It reveals that the Dupangling biotite monzogranites mainly originated from the partial melting of ancient crustal materials and were mixed with a certain degree of depleted mantle materials.The metamorphic peak period of the Indosinian movement is from 258 Ma to 243 Ma.South China was in an extensional tectonic setting after 233 Ma and mantle-derived basalt magma was in a wide range of underplating,which caused the crustal material remelting to form the Dupangling Indosinian aluminous Atype granite(rapakiwi granite)under an extensional background.