K-Ar Ages and Geochemical + Sr-Nd Isotopic Compositions of Adakitic Volcanic Rocks, Western Shandong Province, Eastern China: Foundering of the Lower Continental Crust

An older oversaturated monzonitic and a younger undersaturated syenitic suite of Cretaceous igneous rocks are present at Commerce Mountain and adjacent areas of the southern Canadian Rockies. The undersaturated suite consists of foid-bearing alkali feldspar syenite and trachyte, with lesser amounts of phonolite and foid syenite. Normative data indicate a higher degree of undersaturation attributable to the presence of garnet. Sr and Nd isotopic data are consistent with generation of the magma by partial melting of an enriched source in the upper mantle or the lower continental crust; Pb isotopic data are more consistent with a source in the lower continental crust. High Sr and Ba concentrations in analcime-bearing alkali feldspar megacrysts support early crystallization at high pressure. Replacement of the analcime by muscovite suggests emplacement and continued crystallization at shallow depth under low pressures, at which the analcime was unstable. Prior to emplacement in the upper crust, the Commerce Mountain suite evolved in composition from mafic analcime-bearing perthite syenite to felsic analcime-nepheline-bearing perthite syenite to nepheline (micro)monzosyenite and to analcime (micro)syenite compositionally similar to analcime phonolites found in adjacent areas. Extensive metamorphic-metasomatic alteration suggests that Commerce Mountain was a volcanic center, possibly associated with the eruption of the Crowsnest Formation volcanic rocks.

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Microcontinent subduction and S-type volcanism prior to India–Asia collision

Scientific Reports ◽

10.1038/s41598-021-94492-y ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Zongyao Yang ◽

Juxing Tang ◽

M. Santosh ◽

Xiaoyan Zhao ◽

Xinghai Lang ◽

...

Keyword(s):

Continental Crust ◽

Late Stage ◽

Volcanic Rocks ◽

Isotope Ratios ◽

Lower Continental Crust ◽

Nd Isotope ◽

Magmatic Rocks ◽

Upper Continental Crust ◽

Volcanic Succession

AbstractContinental crust has long been considered too buoyant to be subducted beneath another continent, although geophysical evidence in collision zones predict continental crust subduction. This is particularly significant where upper continental crust is detached allowing the lower continental crust to subduct, albeit the mechanism of such subduction and recycling of the upper continental crust remain poorly understood. Here, we investigate Paleocene S-type magmatic and volcanic rocks from the Linzizong volcanic succession in the southern Lhasa block of Tibet. These rocks exhibit highly enriched 87Sr/86Sr, 207Pb/206Pb and 208Pb/206Pb together with depleted 143Nd/144Nd isotope ratios. The geochemical and isotopic features of these rocks are consistent with those of modern upper continental crust. We conclude that these Paleocene S-type volcanic and magmatic rocks originated from the melting of the upper continental crust from microcontinent subduction during the late stage of India–Asia convergence.

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