Occurrence of Alaskan‐type mafic‐ultramafic intrusions in the North Qilian Mountains, northwest China: Evidence of Cambrian arc magmatism on the Qilian Block

Abstract

Field relationships, mineralogy and petrology, whole‐rock chemistry, and age of the Zhamashi mafic–ultramafic intrusion in the North Qilian Mountains, northwest China, have been studied in the present work. The Zhamashi intrusive body consists of ultramafic, gabbroic, and dioritic rocks in a crudely concentrically zoned structure. The ultramafic rocks are layered cumulates with rock types varying continuously from dunite through wehrlite and olivine clinopyroxenite to clinopyroxenite. The gabbroic and dioritic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to diorite. The ultramafic and adjoining gabbroic rocks are discontinuous in lithology and discordant in structure across the interface. The interface is steep, sharp, and fractured. Contact metamorphic zones are well developed between the Zhamashi intrusive body and the country rock. The concentrically zoned structure of the intrusive body and the intrusion into the continental crust are the two main pieces of evidence for considering that the Zhamashi intrusion is Alaskan‐type. The mineral chemistry of the chromian spinels (Cr‐spinels) and clinopyroxenes, and the variation trend of the whole‐rock compositional plot in the (Na2O + K2O)–FeO–MgO (AFM) diagram are also supportive of this consideration. The age of the Zhamashi intrusive body, determined with sensitive high mass‐resolution ion microprobe on the zircon grains, is 513.0 ± 4.5 Ma. Parental magma of the Zhamashi intrusion is compositionally close to the primitive magma produced by partial melting of the mantle peridotite. It was differentiated by fractional crystallization at low total pressure and under H2O‐rich conditions in an arc environment to form all the major rock types. The concentrically zoned structure of the Zhamashi intrusive body was constructed in two stages: formation of a stratiform‐type layered sequence, followed by diapiric re‐emplacement. The occurrence of the Alaskan‐type intrusion suggests an active continental margin and Cambrian arc magmatism for the northern margin of the Qilian Block.

From Subduction to Collision in the Northern Tibetan Plateau: Evidence from the Early Silurian Clastic Rocks, Northwestern China

“…In the Qilian Orogen, three potential Cr-spinel sources are present: the Alaskan-type Zhamashi mafic intrusions along the north margin of the Central Qilian Block (Tseng et al 2009), the Ordovician island-arc basaltic rocks (Hsu and Shau 2002), and the mafic-ultramafic bodies constituting the North Qilian ophiolites (Feng and He 1996;Zhang et al 1998b;Tseng et al 2007;Xia and Song 2010). The low TiO 2 content, high Fe 2ϩ /Fe 3ϩ ratio, low Fe 3ϩ # value, and large grain size of the detrital spinels closely match those of spinels from mantle peridotites ( fig.…”

“…f, TiO 2 (wt%) versus Al 2 O 3 (wt%); fields after Kamenetsky et al (2001). Shaded fields in c-f: A p Dongcaohe ophiolite data from Tseng et al (2007); B p North Qilian arc volcanics data from Tseng et al (2009); C p Zhamashi intrusion data from Hsu and Shau (2002). In e and f, ARC p island-arc basalt; CFB p continental flood basalt; LIP p large igneous province; MORB p mid-ocean ridge basalt; OIB p ocean-island basalt; SSZ p suprasubduction zone.…”

“…Therefore, the only possible spinel source is the ophiolitic rocks in the North Qilian Belt ( fig. 3), precluding a derivation from the Zhamashi intrusions (Tseng et al 2009) or the Ordovician islandarc basaltic rocks (Hsu and Shau 2002).…”

“…These basement rocks have a Yangtze (Gondwana) affinity based on geochronology, geochemistry, and paleontology (Zhang et al 1998aGuo et al 2000;Wan et al 2000Wan et al , 2006Gehrels et al 2003a;Tung et al 2007a;Xu et al 2006Xu et al , 2007Yong et al 2008a). The Neoproterozoic basement assemblage within the belt underwent early Paleozoic orogeny (BGMRQP 1991;Fan and Lei 2007;He et al 2007;Chen et al 2008;Yong et al 2008b;Tseng et al 2009). Paleozoic granitic intrusions within the belt can be grouped into subduction-related I-type, syncollisional S-type, and postcollisional I-type on the basis of geochemistry, and they range in age from 488 to 382 Ma (fig.…”

Zircon U–Pb geochronology, geochemistry, and Sr–Nd isotopes of the Ural–Alaskan type Tuerkubantao mafic–ultramafic intrusion in southern Altai orogen, China: Petrogenesis and tectonic implications

“…They are restricted to late-Precambrian and Phanerozoic orogenic belts, such as those in the Cordillera of North and South America (Taylor, 1967;Irvine, 1974;Tistl et al, 1994;Pettigrew and Hattori, 2006), Urals and Koryak (Fershtater et al, 1997;Batanova et al, 2005), New South Wales (Johan, 2002), Southland of New Zealand (Spandler et al, 2000(Spandler et al, , 2003, the Eastern Desert of Egypt (Helmy and El Mahallawi, 2003), and along the East African orogenic zone in western Ethiopia (Grenne et al, 2003). They have also been found in China in the last few decades Tseng et al, 2009;Su et al, 2012;Deng et al, 2015).…”