塔里木板块东北边缘弧后裂谷系统基性、超基性岩特征

The Characteristics of basic-ultrabasic rocks in the back-arc rift system on the northeastern margin of the Tarim plate

伸展系统中的基性、超基性岩与铜镍硫化物矿床密切相关.本文研究了塔里木板块东北边缘(可追溯到中元古代)的基性、超基性岩的岩石学、地球化学特征及其形成构造环境,认为该区的基性、超基性杂岩可分为两类:一类形成于裂谷作用早期(中元古代),另一类形成于裂谷作用鼎盛时期或弧后盆地阶段(早古生代).后者可归入蛇绿杂岩中;前者与镍铜成矿系统密切相关,但很难完全归入已有的基性、超基性岩分类之中,属于一种新类型,建议将其命名为大陆边缘裂谷型基性超基性岩.研究区大陆边缘裂谷型基性超基性岩Sm-Nd模式年龄为1 026～1 749Ma,其深成岩(黑山岩体)岩石组合为:石英正长岩-辉长岩-角闪辉长岩-含云母斜长角闪辉橄岩-含云母角闪辉橄岩-含云母斜长角闪橄榄岩-辉石岩,超基性岩m/f平均值为5.5,∑REE为20.9×10-6～64.71×10-6,平均29.91,LREE略富集,辉长岩TiO2为0.92%;浅成岩(黑山北岩体)岩石组合为:灰黑色细粒角闪辉长(玢)岩-暗绿黑色细粒绿泥石化辉长苏长岩-深黑色中粒橄榄二辉岩,TiO2强烈富集,其中辉长(玢)岩TiO2为1.32%～3.32%,MgO、CaO亏损,Rb、Sr、Y、Zr、Nb、Ba、Cs、Hf、Ta、Th、U强烈富集,Sc、V、Cu较富集,Cr、Co、Zn亏损,LREE略富集.表明中元古代塔里木板块东北边缘已经出现活动大陆边缘后方伸展体制,在该体制下岩浆多期次脱离地幔而上侵.

Basic-ultrabasic rocks in an extensional regime are closely related to copper-nickel deposits. The geochemistry and petrology of basic and ultrabasic rocks on the northeastern margin of the Tarim plate (whose age may be traced back to Mesoproterozoic) and their tectonic setting are studied in this paper. The authors consider that the basic-ultrabasic complex in this area may be classified into two types. One type formed in the early stage of rifting (Mesoproterozoic) and the other type formed in the culmination of rifting or back-arc basin stage (Paleoproterozoic). The latter may be assigned to the ophiolitic complex category, while the former is closely related to the nickel-copper metallogenic system, but difficult to assign to any of the available basic-ultrabasic categories, which is a new type. The authors suggest that the former type be named continental-margin rift type basic-ultrabasic rocks. The Sm-Nd model age of this type of rock in this area is TDM=1 026-1 749 Ma. The association of its plutonic rocks (Heishan complex) is quartz syenite-gabbro-hornblende gabbro-mica-bearing plagioclase-hornblende harzburgite-mica-bearing hornblende harzburgite-mica-bearing plagioclase-hornblende peridotite-pyroxenite. The average m/f ratio of the ultrabasic rocks is 5.5 and ∑REE is 20.9×10-6 to 64.71×10-6, with an average of 29.91×10-6, and the LREE is slightly enriched. The TiO2 content of the gabbros is 0.92%. The association of their hypabyssal rocks (North Heishan complex) is grayish black, fine granular hornblende gabbro (gabbro porphyrite)-dark greenish black, finely granular chloritized gabbro norite-deep black medium-grained olivine websterite. Their TiO2 content is strongly enriched . In hornblende gabbro (gabbro porphyrite),TiO2 is 1.32%-3.32%; MgO and CaO are depleted; Rb, Sr, Y, Zr, Nb, Ba, Cs, Hf, Ta, Th and U are strongly enriched; Sc, V and Cu are slightly enriched; Cr, Co and Zn are depleted; and LREE is slightly enriched. All these indicate that a back-arc extensional regime had already appeared on the northeast margin of the Tarim plate in the Mesoproterozoic, and that magmas were separated from the mantle multiple times under this regime.