Geochronology,geochemistry and geological significance of multi-porphyritic alkaline basalt from Hala'alate Formation in Urho area,West Junggar,Xinjiang
Geochronology,geochemistry and geological significance of multi-porphyritic alkaline basalt from Hala'alate Formation in Urho area,West Junggar,Xinjiang
A suite of porphyritic-rich basalt recently unearthed at the base of the Hala'alate Formation to the north of Urho Town in Xinjiang.It exhibits phenocryst predominantly composed of plagioclase,whose contents occupy the total of the basalt of 30％～40％.The crystals display a robust,coarse structure with well-preserved cluster twin-crystals,showcasing a particle size range of 8 ×0.3 mm～18 ×3mm.The overall thickness of this porphyritic-rich basalt exceeds 90m.Basalts of such considerable thickness,characterized by stability and distinctive lithological features,are exceedingly uncommon in this region and even throughout the northern Xinjiang,with scant documented occurrences.The findings indicate that the porphyry in the porphyritic-rich basalt of the Hala'alate Formation primarily consists of light reddish-brown and grayish-white feldspar.Electron microprobe analysis identifies the light reddish-brown feldspar as albite and andesine.LA-ICP-MS zircon U-Pb dating places the rocks in the Late Carboniferous Epoch at 306±2Ma.Geochemical analyses reveal their classification in the alkaline sodium series,showing low SiO2 content(48.84％～49.63％),high Al2O3(17.78％～18.78％),MgO(4.59％～9.29％),and total alkali content(6.62％～7.11％).They are sodium-rich and potassium-poor(K2O/Na2O ratio=0.16～0.20).There is a relative enrichment of light rare earth elements and a depletion of heavy rare earth elements.Large ion lithophile elements such as Rb,Ba,K and Sr are enriched,while high field strength elements such as Th,U,Nb and Ta are depleted.The rocks exhibit a positive εHf(t)value range of 9.94～14.86 and a younger two-stage Hf model tDM2 age range of 0.37～0.68Ga.In summary,the porphyritic-rich basalt is considered to possess dual geochemical characteristics of N-MORB and arc magmatic series,formed through partial melting of garnet and spinel lherzolite from the depleted mantle source with fluid metasomatism.The geochemical characteristics of"proto-arc"of"Nb,Ta depletion but Ti non-depletion"exhibited by the multi-porphyritic alkaline basalt indicate that the Bagutu tectonic belt entered a new arc-basin tectonic system during the Late Carboniferous.The multi-porphyritic basalt of the newborn initial arc at the bottom of the Hala'alate Formation and the mature arc adakite-like volcanic rocks above it record a relatively complete history of the formation and evolution of the accretionary arc volcanic rocks.
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