LATE CARBONIFEROUS VOLCANIC ACTIVITY AND ITS INFLUENCE ON SEDIMENTARY ENVIRONMENT IN THE SOUTHERN MARGIN OF TUR-PAN-HAMI BASIN
In Tainan Sag of Turpan-Hami Basin,there were plenty layers of volcanic rocks and volcani-clastic layers formed during the sedimentary period of the Carboniferous Decaner Formation,and the influ-ence of volcanic activity on the sedimentary environment is not clear.This investigation utilizes data from 7 drilling and logging wells,microscopic thin section observations of 49 samples,and analyses of major and trace elements within the rocks to unravel their influence on sediments.Geochemical analysis reveals that the volcanic rocks of the Dikaner Formation exhibit high SiO2 content(＞56％),along with moderate to strong negative Eu anomalies(δEu=0.36-0.79,average 0.64),an A/CNK value of less than 1.1 and(La/Yb)N ratios ranging from 1.84 to 5.84,indicative of a dacitic-rhyolitic composition.These rocks show a depletion in high field strength elements such as Nb,Ta,and Ti,similar to continental arc mag-matic rocks,suggesting their origin from partial melting of metabasic rocks in the lower continental crust.The Dikaner Formation contains numerous medium-acid pyroclasts whose major elements overlap with volcanic rock samples.These pyroclasts exhibit varying SiO2 content(54.09-75.55％)and ∑REE ran-ging from 85.5× 10-6 to 125.0× 10-6,with a weak to medium negative Eu anomaly(δEu=0.57-0.80,mean 0.67).Based on the mantle-normalized model,these pyroclastic rocks share geochemical character-istics similar to the middle-upper continental crust,suggesting they are predominantly magmatic clastic sources within the medium-acid volcanic arc.The tectonic discrimination diagram for the Dikaner Forma-tion indicates that the detrital rocks were deposited in a continental island arc setting,consistent with the volcanic rocks of the same period.It is inferred that both the volcanic and clastic rocks of the Dikaner For-mation were formed during the subduction of the North Tianshan Ocean beneath the Turpan-Hami Basin.Subduction fluids and mantle heat triggered extensive partial melting of the lower continental crust,lead-ing to intense volcanic activity and a gradual increase in the source components of meso-basic pyroclastic rocks from the bottom to the top of the Dikaner Formation.Analysis of indicators from Dikaner Formation mudstones suggests that during the Upper Carboniferous sedimentary period,paleoclimate conditions transitioned from dry to wet.Enhanced chemical weathering in the volcanic source area increased paleo-sa-linity in the sedimentary environment,promoting reducing conditions that negatively impacted primary productivity but facilitated organic matter preservation.
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