Tectonic affinity and evolution of the Alxa Block during the Neoproterozoic: Constraints from zircon U-Pb dating, trace elements, and Hf isotopic composition

The Jinchuan deposit, which is located in the Longshoushan uplift (LU) on the southwest margin of the Alxa Block (AB), is the third largest Ni-Cu deposit in the world. There are igneous rocks in the LU that widely developed in similar to 830 Ma, which include ore-bearing ultramafic rock and ore-poor mafic-ultramafic rocks. These rocks were produced in an intracontinental extension-rift environment, which is likely related to the Rodinia supercontinent (RS) breakup. However, contemporary magmatic rocks are absent in the AB, except the similar to 830 Ma igneous rocks in the LU. In this study, we present two types of metamorphic mafic-ultramafic rocks at the border of Jinchuan II and IV mineral areas, which is inside the F-17 fault zone. One type is a red-coloured altered rock formed by ultramafic rock oxidation; there are a small number of zircons with multiple ages in the rock, which are considered to be inherited and metamorphic (hydrothermal) zircons. The other type is a chlorite tremolite schist formed by the metamorphism of fault clastics; there are a large number of zircons developed in this rock, which are mostly magmatic zircons with ages between 900 and 800 Ma and a few hydrothermal zircons. Analysis of the trace elements, the 900-800 Ma zircons, have high (Sm/La)(N), low Ce/Ce*, and high Th/U values, which fit the characteristics of typical magmatic zircons. There are also high Y, U, HREE, Nb, and Ta values, which indicates that the host rocks were mostly larvikite and dolerite, and high Y, Ce, Yb, Th, Gd, and Er values indicate a within-plate basaltic origin. All these 900-800 Ma zircons have a epsilon Hf (t) in the range of -2 to -7, which is similar to the Jinchuan ore-bearing ultramafic rock with a partial melting origin of enriched mantle. The absence of detrital zircon age peaks in the Neoproterozoic indicates that the AB has been separated from the North China Craton since the Neoproterozoic to Paleozoic. The detrital zircons in the Yangtze Block (YB) strata with Neoproterozoic ages are slightly younger than those in the AB, with more depleted epsilon Hf (t) values, which is similar to the zircons from the Jiangnan Orogen igneous rocks. All this evidence indicates that the AB may have experienced at least 100 Myr of intracontinental extension-rift environment from 900 to 800 Ma during the Neoproterozoic and participated in the breakup of the RS. However, the location of the AB during the Rodinia may not have been beside the YB.