Characteristics and tectonic significance of the late proterozoic Aksu blueschists and diabasic dikes, northwest Xinjiang, China

J. G. Liou, S. A. Graham, S. Maruyama, R. Y. Zhang

Research output: Contribution to journalArticle

68 Citations (Scopus)

Abstract

A stratigraphically coherent blueschist terrane near Aksu in northwestern China is unconformably overlain by unmetamorphosed sedimentary rocks of Sinian age (∼600 to 800 Ma). The pre-Sinian metamorphic rocks, termed the Aksu Group, were derived from shales, sandstones, basaltic volcanic rocks, and minor cherty sediments. They have undergone multi-stage deformation and transitional blueschist/greenschist-facies metamorphism, and consist of strongly foliated chlorite-stilpnomelane-graphite schist, stilpnomelane-phengite psammitic schist, greenschist, blueschist, and minor quartzite, metachert, and meta-ironstone. Metamorphic minerals of basaltic blueschists include crossitic amphibole, epidote, chlorite, albite, quartz, and actinolite. Mineral parageneses and compositions of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and 300 to 400° C. Many thin diabasic dikes cut the Aksu Group; they are characterized by high alkali, TiO2, and P2O5, contents and possess geochemical characteristics of within-plate basalts; some of these diabasic rocks contain sodic clinopyroxene and amphibole as primary phases and have minor pumpellyite, albite, epidote, chlorite, and calcite as the prehnite/pumpellyite-facies metamorphic assemblage. This prehnite/pumpellvite-facies overprint did not affect the host rocks of the blueschist-facies lithologies. K-Ar and Rb-Sr ages of phengite and whole rocks from pelitic schists are ∼690 to 728 Ma, and a 40Ar/39Ar age of crossite from the blueschist is 754 Ma. The basal conglomerate of the overlying Sinian to Eocambrian sedimentary succession contains clasts of both the blueschist and cross-cutting dike rocks, clearly demonstrating that conditions required for blueschist-facies metamorphism were attained and ceased at least 700 Ma. The northward-increasing metamorphic grade of the small blueschist terrane may reflect northward subduction of an accretionary complex beyond the northern edge of the Tarim craton. Abundant subparallel diabasic dikes indicate a subsequent period of Pre-Sinian rifting and diabasic intrusion along the northern margin of Tarim; a Sinian siliciclastic and carbonate sequence was deposited unconformably atop the Aksu Group and associated diabase dikes.

Original languageEnglish
Pages (from-to)228-244
Number of pages17
JournalInternational Geology Review
Volume38
Issue number3
Publication statusPublished - Mar 1996
Externally publishedYes

Fingerprint

blueschist
Sinian
dike
Proterozoic
tectonics
blueschist facies
amphibole
schist
chlorite
pumpellyite
prehnite
phengite
epidote
albite
terrane
metamorphism
crossite
rock
metamorphic facies
ironstone

ASJC Scopus subject areas

  • Geology

Cite this

Characteristics and tectonic significance of the late proterozoic Aksu blueschists and diabasic dikes, northwest Xinjiang, China. / Liou, J. G.; Graham, S. A.; Maruyama, S.; Zhang, R. Y.

In: International Geology Review, Vol. 38, No. 3, 03.1996, p. 228-244.

Research output: Contribution to journalArticle

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