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

Abstract：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.