闽西北罗古岩岩体印支期年龄的厘定及其岩石成因和构造意义

Discovery of an Indosinian Granite: Luoguyan Pluton in Northwestern Fujian Province, South China, and Its Petrogenesis and Tectonic Significance

以闽西北地区的罗古岩花岗岩岩体为研究对象,系统研究了它的年代学、全岩地球化学和Sr-Nd-Hf同位素地球化学,并据此探讨了该岩体的成因。LA-ICP-MS锆石U-Pb定年结果表明,罗古岩花岗岩岩体两个样品(LGY02和LGY10)的年龄为(227±3)Ma(LGY02)和(234±2)Ma(LGY10),指示该花岗岩体为印支期岩浆活动的产物。岩石地球化学组成上表现为高硅(SiO2>74%)、富碱(K2O+Na2O=8.57%~9.22%,K2O/Na2O>1.4)、准铝(A/CNK值多数小于1),贫钙、镁和铁。稀土元素总量较低(116.72×10-6~236.53×10-6),铕负异常强烈(δEu=0.17~0.37)。富集Rb,Nd,Th,U和Hf等元素,而Ba,Nb,Sr,Zr和Ti显示为负异常。Zr+Y+Ce+Nb(217.5×10-6~332.8×10-6)含量低于A型花岗岩的下限值(350×10-6)。综合岩相学和岩石地球化学特征指示该岩体应属高分异的I型花岗岩。同位素组成上,(87Sr/86Sr)i位于0.7078~0.7130之间,εNd(t)值较低(-12.17^-10.70),两阶段Nd模式年龄为1.87~2.00 Ga;岩浆锆石的εHF(t)值也较低但有较大的变化范围(-13.0^-6.8),两阶段Hf模式年龄为1.69~2.08 Ga。结合印支期同处于浦城—武平断裂带上的洋坊正长岩和闽西北地区夏茂暗色岩脉的富集地幔组分的发现,本文认为罗古岩岩体是由类似于夏茂暗色岩脉的富集地幔组分与基底麻源群的熔融组分在伸展拉张的构造背景下混合而成,并经历了高程度的结晶分异作用。

In this paper, the Luoguyan pluton in the northwestern inland area of Fujian Province was selected as an example for a detailed geochronological, whole rock geochemical, and Sr-Nd-Hf isotopic geochemical study in order to elucidate its petrogenesis. LA-ICP-MS U-Pb zircon ages of two samples （LGY02 and LGY10） from the Luoguyan pluton are （227.1 ± 2.6） Ma （LGY02） and （234.4 ± 1.5） Ma （LGY10）, indicating that this pluton was generated in the Indosinian stage. Chemically, this pluton is metaluminous （most of the A/CNK values are lower than 1.0）, and shows high contents of silica （SIO2〉74%） and alkalies （K20＋Na20 =8.57%-9.22%,K2O/Na2O〉1.4）, low abundance of calcium, magnesium, and iron. The rocks display low total REE contents （∑REE=116.72 × 10^-6-236.53 × 10^-6） and low contents of Zr＋Ce＋Nb＋Y （〈 350 ×10^-6） with strongly negative Eu anomalies （δEu=0.17-0.37）, and are enriched in Rb, Nd, Th, U, Hf, depeted in Ba, Nb, Sr, Zr, Ti. Overall, our integrated mineralogical and geochemical features suggest that the Luoguyan granites are highly fractionated I-type granites. Isotopically, samples show high and variable initial ^87Sr/^86Sr ratios （0.7078±0.7130）, yet low and constant aNd（t） （--12.17-10.70）, corresponding to two stage model age of the 1.87-2.00 Ga. However, their zircon Hf isotopes show large variations, with εHF（t） values varying between -13.0-6.8, corresponding to two stage model age of 1.69-2.08 Ga. Combining with the discovery of the Indosinian Yangfang syenite located in Pucheng-Wuping fault zone and the Xiamao diabase originated from the enriched mantle, we suggest that the Luoguyuan pluton was derived from a mixed magma which mingled with an enriched mantle-derived component similar to the Xiamao diabase with a crustal felsic component derived from the partial melting of basement Mayuan metamorphic rocks, during which the tectonic setting was an extension environment, and then experienced a high degree of differentiation during crystallization.