Sources and Petrogenesis of Late Triassic Dolerite Dikes in the Liaodong Peninsula: Implications for Post-collisional Lithosphere Thinning of the Eastern North China Craton

Abstract

A combination of major and trace element, whole-rock Sr, Nd and Hf isotope, and zircon U^Pb isotopic data are reported for a suite of dolerite dikes from the Liaodong Peninsula in the northeastern North China Craton. The study aimed to investigate the source, petrogenesis and tectonic setting of the dikes. Sensitive high-resolution ion microprobe U^Pb zircon analyses yield a LateTriassic emplacement age of $213 Ma for these dikes, post-dating the collision between the North China and Yangtze cratons and consequent ultrahigh-pressure metamorphism. Three geochemical groups of dikes have been identified in the Liaodong Peninsula based on their geochemical and Sr^Nd^Hf isotope characteristics. Group 1 dikes are tholeiitic, with high TiO 2 and total Fe 2 O 3 and low MgO contents, absent to weak negative Nb and Ta anomalies, variable ( 87 Sr/ 86 Sr) i (0Á7060^0Á7153), " Nd (t) (À0Á8 to À6Á5) and " Hf (t) (À2Á7 to À7Á8) values, and negative Á" Hf (t) (À1Á1 to À7Á8).They are inferred to be derived from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with some upper crustal assimilation and fractional crystallization. Group 2 dikes have geochemical features of high-Mg andesites with ( 87 Sr/ 86 Sr) i values of 0Á7063^0Á7072, and negative " Nd (t) (À3Á0 to À9Á5) and " Hf (t) (À3Á2 to À10Á1) values, and may have originated as melts of foundered lower crust, with subsequent interaction with mantle peridotite. Group 3 dikes are shoshonitic in composition with relatively low ( 87 Sr/ 86 Sr) i values (0Á7061^0Á7063), and negative " Nd (t) (À13Á2 to À13Á4) and " Hf (t) (À11Á0 to À11Á5) values, and were derived by partial melting of an ancient, re-enriched, refractory lithospheric mantle in the garnet stability field. The geochemical and geochronological data presented here indicate that Late Triassic magmatism occurred in an extensional setting, most probably related to post-orogenic lithospheric delamination.

Destruction of North China Craton: Insights from temporal and spatial evolution of the proto-basins and magmatism

“…Triassic magmatism of the North China Craton was concentrated in the Late Triassic, and mostly occurred on the margins of the craton (Figure 2(b)), such as the calc-alkaline granite in the Qinling Orogen [38], syenites in the Jiazishan (Shandong), alkaline intrusions in Liaodong Peninsula and southern Jilin Province [7,[39][40][41] and some maficultramafic complex on the northern margin of the North China Craton [6]. The asthenosphere component was discriminated in all magmatism surrounding the North China Craton during the Late Triassic, all attributed to the lithospheric delamination in post-collision setting [6,7,[38][39][40][41]. No matter what mechanisms, there is a notable asthenosphere upwelling surrounding the North China Craton during the Late Triassic.…”

“…abundant materials of recycled earlier sediments from the eastern North China Craton in the Late Triassic (unpublished data). Varying degrees of asthenosphere upwelling on the edges of the North China Craton occurred in the Late Triassic [6,7,38], indicating that the edges of the North China Craton started to be destructed at that time. No proof supports the superficial differentiation within the eastern North China Craton during the Late Triassic at present, or because the Triassic strata were denudated, the proofs were destroyed.…”

“…Gao et al [3] suggested that the Jurassic high-Mg adakitic rocks of the western Liaoning Province were derived by partial melting of delaminated crust, then proposed the delamination of the North China Craton during the Middle Jurassic. But Yang et al [7] inferred the delamination during the Late Triassic (~210 Ma) by studying the dolerite in Liaodong Peninsular. Jiang et al [8] suggested that the lithospheric thinning would have started in the Late Jurassic according to the studies on the lamprophyre in Liaodong Peninsular.…”

Rapid lithospheric thinning of the North China Craton: New evidence from cretaceous mafic dikes in the Jiaodong Peninsula

“…Thus, the diagnostic trace element ratios of the Triassic "Group 1" rocks from Liaodong Peninsula average Nb/U = 31 ± 5 and Ce/Pb = 11 ± 3 and are thus significantly lower than the canonical mantle values of Nb/U = 47 ± 10 and Ce/Pb = 25 ± 6 (Hofmann et al, 1986). Yang et al (2007) interpret these ratios as having been lowered from normal asthenospheric mantle values by crustal contamination. Although this is a reasonable interpretation, the involvement of a source with more lithospheric character seems equally likely, and in this sense the geochemical signals from the Triassic rocks are not clearly diagnostic.…”

Petrogenesis of a Late Carboniferous mafic dike–granitoid association in the western Tianshan: Response to the geodynamics of oceanic subduction

“…It is generally accepted that mafic dike-granite associations were emplaced in extensional tectonic regimes linked to post-collisional or intraplate extensional settings (Eby, 1992;Said and Kerrich, 2010;Xu et al, 2008;Yang et al, 2007b). However, the genetic relationship between granites and mafic rocks is a matter of much current debate.…”