东昆仑大格勒泥盆纪正长岩岩石成因和地质意义

Petrogenesis and Geological Significance of Devonian Syenites in the Dagela Area,East Kunlun

东昆仑大格勒地区发现了与铌、稀土共生的碱性岩-碳酸岩杂岩体,表明东昆仑造山带具有碱性岩-碳酸岩型稀有金属矿床的找矿前景。团队对大格勒杂岩体的岩石组合、岩石地球化特征、岩浆演化及构造背景等问题开展了一系列研究。最近新发现了与碱性镁铁-超镁铁质岩、碳酸岩杂岩形成时代相近、空间上相伴共生的正长岩体,为深入了解大格勒杂岩体提供了新视角。大格勒岩体主体为角闪正长岩,少量为石英正长岩,主要矿物为正长石,次为更长石和铁浅闪石,少量为钠长石、石英、钙铁辉石、铁橄榄石和铁叶云母。锆石U-Pb定年结果表明角闪正长岩形成于407~402 Ma。主量元素组成显示,角闪正长岩的SiO_(2)(59.08%~63.14%)含量中等,Al_(2)O_(3)(16.16%~17.23%)、K_(2)O+Na_(2)O(9.54%~11.06%)含量高,富钾(K_(2)O/Na_(2)O=1.45~2.02),极富铁(Fe O^(T)/(MgO+Fe O^(T))=0.93~0.94),为准铝质、钾玄岩系列;微量元素富集Hf(18.6×10^(-6)~30.6×10^(-6))、Ba(937×10^(-6)~1386×10^(-6)),极富Zr(1017×10^(-6)~1842×10^(-6)),但亏损Nb、Ta等,Eu异常不明显-弱Eu正异常(δEu=0.94~1.35,平均值为1.14)。大格勒正长岩具富集的Nd同位素(ε_(Nd)(t)为-3.9~-3.4)和高放射性Pb同位素组成(^(208)Pb/^(204)Pb(t)=38.164~38.232,^(207)Pb/^(204)Pb(t)=15.636~15.640,^(206)Pb/204Pb(t)=18.321~18.400),其锆石Hf同位素组成较均一(εHf(t)=-1.57~1.18)。结合区域构造背景,认为大格勒正长岩形成于碰撞后岩石圈伸展环境,角闪正长岩可能由富集地幔熔融产生的岩浆经过分离结晶而成。

Alkaline-carbonatite complex associated with Nb and rare earth element mineralization has been discovered recently in the Dagela area,indicating the prospective potential of the carbonatite and alkaline rock-type rare metal ore deposits in the East Kunlun orogenic belt.However,the rock assemblages,magma evolution,and tectonic setting of the Dagela complex have not been well-documented.The recently discovered syenite intrusions in the Dagela area provide an important opportunity for solving these problems.The Dagela syenite intrusions mainly consist of hornblende syenites,with minor amounts of quartz syenites.These rocks are composed of orthoclase,oligoclase,and ferrosilite,with minor amounts of albite,quartz,calcic ferroaugite,ferro-olivine,and siderophyllite.Zircon U-Pb dating indicates that the hornblende syenites were generated at 407 Ma to 402 Ma.The hornblende syenites have medium SiO_(2)(59.08%−63.14%),high Al_(2)O_(3)(16.16%−17.23%)and K_(2)O+Na_(2)O(9.54%−11.06%)contents,with K_(2)O/Na_(2)O of 1.45 to 2.02,indicating metaluminous and shoshonitic characteristics.They are extremely enriched in iron(FeO^(T)/MgO=12.46−15.84,FeO^(T)/(MgO+FeO^(T))=0.93−0.94)and Zr(1017×10^(-6)−1842×10^(-6)),with high Hf(18.6×10^(-6)−30.6×10^(-6))and Ba(937×10^(-6)−1386×10^(-6))contents,and depleted in Nb and Ta,with negligible-slightly positive europium anomalies(δEu=0.94−1.35,average value is 1.14).They have enriched whole-rock Nd isotopic compositions withε_(Nd)(t)values ranging from−3.9 to−3.4,and high radiogenic whole-rock Pb isotope compositions with^(208)Pb/^(204)Pb(t)ranging from 38.164 to 38.232,^(207)Pb/^(204)Pb(t)ranging from 15.636 to 15.640,and^(206)Pb/^(204)Pb(t)ranging from 18.321 to 18.400.Their zircon Hf isotopic compositions are homogeneous(εHf(t)=−1.57−1.18).The Dagele hornblende syenites were most likely generated by fractional crystallization of magmas derived from partial melting of enriched mantle.The hornblende syenites was formed in a reduced environment with low oxygen fugacity(lgfO2 between−17.26 and−15.50).Taking into account the occurrence of the Early to Middle Devonian(418−387 Ma)mafic-ultramafic rocks and Silurian-Early Devonian(430−410 Ma)eclogites in the region,we suggest that the Dagela syenites were possibly generated in a post-collisional extensional tectonic setting.