Geochemical and structural constraints on the magmatic history of the Chandman Massif of the eastern Mongolian Altay Range, SW Mongolia

Abstract

In the Mongolian Altay range, immediately south of the Bogd fault zone, the Chandman Massif intruded the Chandman Khayrkhan Crystalline Complex to the NE. This complex consists of migmatized biotite gneisses, orthogneisses and amphibolites. To the south, the Massif cuts the chlorite schists and quartzites of the Tugrug Formation. The Massif mainly consists of diorites to granites with rare gabbro bodies interspersed with metamorphic host rock screens, generally of amphibolite with calc-silicate lenses. Granitoids are further categorized based on petrography and whole-rock geochemical data into peraluminous and metaluminous groups. Granites comprise the peraluminous group, and possess high (71-77 wt. %) silica contents with abundant modal K-feldspar. Metaluminous granitoids span a wide range in silica contents, from diorite to granite. Also their feldspars are compositionally variable. Both these granitoid units are of volcanic-arc character and display a calc-alkaline geochemical trend. Granodiorites of the metaluminous group contain widespread planar rhythmic schlieren layering. Metaluminous units slightly predate the intrusion of peraluminous granites, but the two suites are occasionally co-magmatic. The Al-in-hornblende barometric analysis combined with plagioclase thermometry reveals a depth of emplacement of 11.5-13.7 km and magma temperatures of 725-775 ºC. Chandman Khayrkhan metamorphic foliations strike NNW. Magmatic fabrics in the Chandman Massif roughly E-W and subparallel E-W solid-state foliations overprint magmatic foliations. All foliations are moderately to steeply dipping. Fabric analysis shows a discordance of structures to the north and south of a fault that cross-cuts the field area. This fault is interpreted to be related to motion on the Bogd Fault. Thus, this area experienced 1) amphibolite-facies metamorphism of the Chandman Khayrkhan Crystalline Complex, 2) exhumation to higher crustal levels, 3) juxtaposition against the greenschist-facies Tugrug Formation, 4) intrusion of the Chandman Massif under tectonic strains that continued through the solidification of plutons, and 5) late block-style rotation related to motion on recent faults. These age and geological constraints identify the Chandman Massif as an intrusion of substantially younger age than the "Caledonian" association into which it was previously placed. It is thus far the only arc-type intrusion in the earliest "Hercynian" age range identified in the Gobi-Altay Terrane. Its metamorphic and magmatic history of migmatization followed by intrusion of metaluminous and peraluminous plutons are similar to those of rocks to the west, in the Tseel Terrane, and may represent its easternmost counterpart. The exact juxtaposition mechanism for metamorphic units of different grade and the formation of schlieren layering in the Chandman Massif granodiorites remain enigmatic.

Geochemical and geomorphological evidence for the provenance of aeolian deposits in the Badain Jaran Desert, northwestern China

“…Bivariate diagrams of trace and rare earth element ratios of sample from the Badain Jaran Desert and the red sand dune field, and compared to their potential source areas. AM field is potential source areas of Altay Mountains and Mongolia Gobi (data from Economos et al, 2008;Hanzl et al, 2008;Zhang et al, 2013); Q field is potential source areas of Qilian Mountains (data from Wan et al, 2006;Yan et al, 2010;Huang et al, 2014). Keys same as for Fig.…”

“…Secondly, runoff volume of the Heihe River drainage is much greater than the ephemeral channels or past perennial (Taylor and McLennan, 1985;Fedo et al, 1995). In (b), the data of granite and granitoid in solid black triangle and star are from Qilian Mountains (Wan et al, 2006;Yan et al, 2010;Huang et al, 2014) and Altay Mountains and Mongolia Gobi (Economos et al, 2008;Hanzl et al, 2008;Zhang et al, 2013), respectively (Data of the potential source rocks are the average values of several samples with large spatial coverage.). Keys same as for Fig.…”

P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

“…The core is 20 km long and 15 km wide, composed mainly of magmatic rocksgranodiorites, diorites and granitesassociated with migmatitic orthogneisses, metasedimentary rocks and amphibolites. The granitoids have I-type calcalkaline chemistry interpreted to reflect a magmatic arc signature (Economos et al, 2008). The depth of magma crystallization was estimated as 11-13 km using Al-inhornblende barometry, and the temperature of solidification was 725-775°C, based on Hbl-Pl thermometry (Economos et al, 2008).…”

“…Despite a large effort during the last decade, the petrological studies carried out in both the Chinese and Mongolian Altai are concentrated mostly on the estimation of peak P–T conditions, retrograde P–T trajectories or geochronology attempting to date paroxysmal stages of metamorphism (Kröner et al ., ; Burenjargal et al ., , ). However, the complexity of the metamorphic and structural evolution of this area (Economos et al ., ; Hrdličková et al ., ; Lehmann et al ., ; Buriánek et al ., ) calls for integrated structural, metamorphic petrology and geochronological studies that may provide information about tectonic history and time‐scales of thermal evolution of the Altai orogeny as well as about mechanisms responsible for exhumation of the high‐grade rocks.…”

Geophysical and geochemical nature of relaminated arc‐derived lower crust underneath oceanic domain in southern Mongolia

“…The Central Asian Orogenic Belt (CAOB) or Altaids is one of the largest accretionary systems of the World covering one third of Asia [ Şengör et al , ]. It is composed of accreted Precambrian continental segments with a strong Neoproterozoic overprint [ Rojas‐Agramonte et al , ], Neoproterozoic ophiolites and accretionary complexes [ Khain et al , ] and Siluro‐Devonian oceanic crust [ Zonenshain et al , ; Kröner et al , ; Safonova et al , ], and Ordovician, Devonian, and Carboniferous arcs and back arcs [ Badarch et al , ; Economos et al , ; Demoux et al , ]. The existing geophysical data [ Mordvinova et al , ; Mordvinova and Artemyev , ; Teng et al , ; Zhang et al , ] show that the thickness of the crust in the southern part of the CAOB is near constant at ~45 km.…”