Search

Garnet peridotites from the Kokchetav ultrahigh-pressure (UHP) massif contain abundant volatile and high field strength element (HFSE) bearing minerals, such as Ti-clinohumite and zircon. These characteristics are interpreted to be related to fluid-infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area. The zircons from the peridotites were dated by using sensitive high-resolution ion microprobe (SHRIMP) and yielded apparent U-Pb ages of 554-494 Ma (weighted mean age, 528 Ma) that are mostly consistent with the timing of UHP metamorphism deduced from diamond-bearing country rocks in this massif. These zircons have an almost flat rare earth element (REE) pattern and very low REE concentrations; these characteristics are similar to those observed in kimberlitic zircons. Inherited zircon cores, although only rarely preserved, yielded apparently Proterozoic ages and have different trace element characteristics compared to the overgrowth rims. These features indicate that the mantle metasomatic events and the recrystallization of Ti-clinohumite and zircon were due to HFSE-enriched fluid infiltration during the UHP metamorphism at great depths. The metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts, because these rocks have characteristically high volatile and HFSE concentrations such as those of the Ti-clinohumite-bearing garnet peridotites. Keywords: mantle metasomatism, Ti-clinohumite, zircon, U-Pb geochronology, ultrahigh-pressure metamorphism.

The northeastern extension of the Altyn Tagh fault, China, beyond the apparently truncated Qilian Shan has been controversial since the recognition of this large left-slip fault. We propose that the Altyn Tagh fault may have been contiguous with the east-northeast to northeast-trending, currently inactive (but apparently active during the Cenozoic) Alxa-East Mongolia fault; the latter truncated the Beishan orogen to the northwest and the Inner Mongolia orogen to the southeast. The Beishan orogen can be correlated with the Inner Mongolia orogen on the basis of the recognition of three offset petrotectonic units and two offset sutures. This correlation requires 400 [+ or -] 50 km of left-lateral offset along the Alxa-East Mongolia fault, identical to the slip along the northern segment of the Altyn Tagh fault inferred by P. Molnar and P. Tapponnier. We suggest a two-stage model for fault evolution: during the first stage, 400 km of displacement separated the Inner Mongolia orogen from the originally contiguous Beishan orogen along a continuous Altyn Tagh-Alxa-East Mongolia fault. During the second stage, the Alxa-East Mongolia fault became inactive, and additional offset along the Altyn Tagh fault has been mainly accommodated by shortening of the Qilian Shah and the Qaidam basin. We infer that the first stage of movement along the Altyn Tagh-Alxa-East Mongolia fault system may have begun around Oligocene time: the region transformed to the second stage of fault movement ca. 13-16 Ma, probably because of thinning of the lithospheric mantle beneath northern Tibet. The proposed model suggests that the Derbur fault in northernmost China and beyond is an extension of the Alxa-East Mongolia limit, and has a history of strike-slip movement. The nearly 400 km of offset on the Alxa-East Mongolia fault may have been accommodated mainly by subduction in the Sea of Okhotsk.

New 40Ar/39Ar ages from rocks in the high-pressure and ultra-high-pressure (HP-UHP) metamorphic complex in the Hong'an and Dabie Mountains areas, east-central China, document a two-phase cooling and exhumation history following Triassic continental collision and metamorphism. Phengite ages from blueschist through kyanite-bearing eclogite facies rocks in Hong'an record initial exhumation from the collision zone between 230 and 195 Ma. Biotite and horn-blende ages from migmatites and eclogite-bearing gneisses from the Dabie Mountains record a cooling event between 128 and 117 Ma, corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons. The Triassic through Early Jurassic 40Ar/39Ar cooling ages corroborate U-Pb and Sm-Nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid. Emplacement of granitic melts within the HP-UHP sequences [approximately] 80 m.y. after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial, rapid exhumation episode.

The largest gabbroic intrusion, Jiaoziyan, in the Northern Orthogneiss of the Dabie Shan was investigated to test whether such intrusions represent arc-related or collision-related magma, as postulated in recent regional tectonic models. The Jiaoziyan intrusions comprise a gabbro-monzogabbro suite and are locally characterized by cumulus textured anhydrous silicates. Petrologic indications against arc magmatism include (1) coexisting olivine-plagioclase compositions beyond the documented range of arc gabbros, and (2) a compositional trend of Al/Ti ratios in the gabbroic clinopyroxenes that is significantly different from subduction-related gabbros worldwide. In contrast, the Jiaoziyan rocks are mineralogically similar to rift gabbros. Geochronologic and geometric arguments against collision-related magmatism (i.e., as a consequence of slab breakoff) include the following: (1) crystallization of the Jiaoziyan mafic intrusion postdated the Triassic (245-240 Ma) subduction and peak orogenic metamorphism by ~100 m.y. and (2) the rocks of the Dabie Cretaceous magmatic suite, including Jiaoziyan, are on the footwall plate, rather than on the hanging wall, of the Triassic subduction-zone framework and do not show a linear distribution. These observations argue that the voluminous Early Cretaceous magmatism in the Northern Orthogneiss is geodynamically unrelated to the Qinling-Dabie orogeny, but was caused by extensional tectonism related to regional magmatic under-plating beneath the thinned Dabie crust. Keywords: gabbros, petrogenesis, extension tectonics, Cretaceous, Dabie Mountains.