Discovery of layered maflc-ultramafic intrusion in Changshagou, Altyn Tagh, and its geological implication:A pilot study on its petrological and geochemical characteristics.

The Changshagou layered intrusion are comprised of some mafic-ultramafic blocks with ages of 462～470 Ma, the Northern Qingshuiquan (NQ) , Southern Qingshuiquan (SQ) and Middle section of Changshagou (MC) , comparted by faults. They intruded in the Neoarchean and Neoproterozoic metamorphic strata on the southern margin of Altyn Tagh. The geochemical characteristics of the mafic-ultramafic rocks from these blocks are not uniform, but they all formed in continental rift tectonic setting according to the enrichments in LREEs and strong incompatible elements and higher Zr/Y ratios(4.1). The NQ mafic-ulmafic block consists of three to four magmatic cycles and exhibits the feature of layered mafic-ultramafic intrusion; higher ( Th/Nb)_N ratios (1.0) , lower Nb/La ratios (1.0) and reverse mineral enwrapping of these rocks indicate the crust contamination and mingling of magma; lower Mg (53. 7～55. 9) of their parental magma suggest they are from evolved magma undergone by significant fractional crystallization. Whereas the lower (Th/Nb)_N ratios (1.0) and higher Nb/La ratios(1.0) of macfic-ultrmafic rocks from SQ and MC indicate they were not contaminated by crust. The increasing FeO~T and TiO_2, and decreasing SiO_2 with decreasing Mg~# of these macfic-ultrmafic rocks from SQ and MC present the distinct Fenner trend for magma evolution. Accordingly, we inferred that the maficultramafic rocks with higher Mg~# (90.6～84.5) from SQ were the magnesium-enriched cumulus phase from the parental ( primary) magma with Mg~# = 78.2, while the mafic-ultramafic rocks with lower Mg~# (75.8～49.2) from MC intrusion containing Ti-magnetite orebodies, were the residual magma phase by Fenner-trend of fractional crystallization from this parental magma. Occurrences of the Changshagou continental rift-type layered mafic-ultramafic intrusion indicate the south margin of Altyn Tagh were in the continental extending setting in middle Ordovician. As a new idea for geologic exploration in this area, the distribution and potential magmatic PGE-Cu-Ni sulfide and V-Ti-Fe oxide metallization of the same type intrusions in this area are worthy of further study.