Geochronology And Geochemistry Of A’re Porphyry In Shangri-la, Northwest Yunnan: Petrogenesis And Implications

The A’re porphyry is located at the south end of the West Porphyry Belt ofZhongdian Arc, the southern part of Yidun Island Arc Belt. It is west to the ZhongzaBlock, southeast to the Yangze Block and east-north to the Ganze—Litang Suture.Based on the detailed field investigation, this paper characterize the A’re porphyryfrom systematic petrography, geochronological, mineral chemical and geochemicaldata. This paper present a revised geodynamic model to explain the magma origin andevolution of A’re porphyry and its implications according to the results and inconjunction with the available information of major ore-bearing porphyry which islocated at the Zhongdian Arc. The differences of magma evolution and thegeodynamic background related to the mineralization between with A’re and Pulang,Xuejiping porphyry also will be given in this paper. The main results are as follows:1) A’re porphyry are composed mainly of quartz diorite porphyry, amphibolediorite porphyry, clinopyroxene diorite porphyry, clinopyroxene-bearing amphibolediorite porphyry. The clinopyroxene-bearing amphibole diorite porphyry hasreplacement remnant texture. The results of LA-ICP-MS zircon U-Pb chronologydisplay the A’re porphyry formed in Late Triassic which is similar to the Indosinianigneous rocks in the Zhongdian arc, and consistent with the magmatism event thatrelated to the Ganze-Litang oceanic crust subduction.2) Mineral chemical results display all of the amphibole phenocrysts aremagmatic calcic amphibole. The amphibole phenocrysts from north end of the A’reporphyry are magnesiobornblende which is characterized with low TFeO （12.99%¹5.51%）, Al₂O₃（5.35⁶.35%） and high SiO₂（44.1⁴8.7%）, Mg#（0.64⁰.73）. The am-phibole phenocrysts from south end of the A’re porphyry are mainly pargasite andcontainning a small amount of iron pargasite and edenite which is characterized withlow SiO₂（37.56⁴5.35%）, Mg#（0.45⁰.61）, high Al₂O₃（7.74¹3.56%）, TFeO （14.58¹8.8%）. The clinopyroxene phenocrysts are diopside except two points whichh fell inthe augite field. The chemical result of clinopyroxene shows that the clinopyroxenecan be divided into diopsid and salite with high Mg#, CaO, Cr₂O₃, low Al₂O₃, Na₂O and high Al₂O₃、Na₂O、low CaO、Cr₂O₃、Mg#respectively. The geothermobarometerresults of amphibole phenocrysts show that the emplacement of magma of the northern A’re porphyry crystallization temperature and pressure is658^℃711and0.14⁰.23GPa respectively, while681⁸26℃and0.35⁰.89GPa for the southernporphyry. The BSE electron images show amphibole oscillatory zoning andclinopyroxene composition zoning in the clinopyroxene-bearing amphibole dioriteporphyry and clinopyroxene diorite porphyry, respectively.3) The A’re porphyry, characterized by a limited range in SiO₂, Al₂O₃content（56.66⁶4.84%and13.90¹7.44%, respectively）, are low Mg#（39.73in average） andcalc-alkaline. These rocks show enrichment in large-ion incompatible elements, anddepletion in high-field strength elements, which are similar with an island arc affinity.It is also shows adakitic affinity with low Y (11.71²1.73×10^-6), Yb (1.20².04×10^-6)and high Sr (490.41²177.81×10^-6) content, Sr/Y (37.44¹17.85×10^-6), La/Yb (21.66³5.53×10^-6) radios. These rocks have⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios of0.705652⁰.706445and0.512395⁰.512413respectively, and initial (⁸⁷Sr/⁸⁶Sr)tratios of0.705155⁰.705463, negative εNd（t） values of-2.2^-1.9, indicate that the magmacannot be derived from the depleted mantle or ancient crust directly.4) The replacement remnant texture in the clinopyroxene-bearing amphibolediorite porphyry is caused by the reaction between water-rich melt and clinopyroxene.And the existence of the amphibole oscillatory zoning and clinopyroxene compositionzoning are caused by the magma mixing. All of the evidence indicate that the existen-ce of the magma mixing process. The geochemical features of the A’re porphyrysuggest the presence of the slab-derived fluids and an insignificant contribution fromsediments in origin. The Sr-Nd isotope mixing modeling results and the trace elementgeochemical characteristics indicate that the magma of A’re porphyry mixing with thecontributions of5¹0%crustal material, the presence of the wall rock xenoliths in theA’re porphyry further proved the role of crustal contamination.5) Based on the geochronology, geochemical characteristics and combined withthe regional tectonic framework and compared with the Pulang, Xujiping porphyry,it is suggested that the A’re porphyry derive from the partial melting of the low-anglesubduction of Ganze-Litang oceanic crust slab and accompanied by the presence ofthe slab-derived fluids, subsequently hybridized by peridotite in the mantle wedge andcrustal contamination and magma mixing. The geochemical characteristics indicate aninsignificant contribution from sediments in origin caused by the scraped off duringsubduction. The difference between Pulang, Xujiping porphyry and A’re porphyry isthe varying degrees of the crustal materials contamination in magma process, and ansignificant contribution from sediments in Pulang porphyry. This difference may beone of the reasons that led to the varying degrees of the metallogenic materials in themagma process and the differences of the mineralization.