ALBERT

Description: The Gongga Shan batholith of eastern Tibet, previously documented as a ca. 32–12.8 Ma granite pluton, shows some of the youngest U-Pb granite crystallization ages recorded from the Tibetan Plateau, with major implications for the tectonothermal history of the region. Field observations indicate that the batholith is composite; some localities show at least seven crosscutting phases of granitoids that range in composition from diorite to leucocratic monzogranite. In this study we present U-Pb ages of zircon and allanite dated by laser ablation–inductively coupled plasma–mass spectrometry on seven samples, to further investigate the chronology of the batholith. The age data constrain two striking tectonic-plutonic events: a complex Triassic–Jurassic (ca. 215–159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14–5 Ma) record of monzonite-leucogranite emplacement. The former age range is attributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduction zone magmatism along the western Yangtze block of south China. The younger component may be related to localized partial melting (muscovite dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, and are among the youngest crustal melt granites exposed on the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of zircons and growth and/or resetting of allanites. The young garnet, muscovite, and biotite leucogranites occur mainly in the central part of the batholith and adjacent to the eastern margin of the batholith at Kangding, where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seismically active strike-slip fault in Tibet and is thought to record the eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites of the Gongga Shan massif and has a major transpressional component in the Kangding-Moxi region. The course of the Xianshui Jiang river is offset by ∼62 km along the Xianshui-he fault and in the Kangding area granites as young as ca. 5 Ma are cut by the fault. Our new geochronological data show that only a part of the Gongga Shan granite batholith is composed of young (Miocene) melt, and we surmise that as most of eastern Tibet is composed of Precambrian–Triassic Indosinian rocks, there is no geological evidence to support regional Cenozoic internal thickening or metamorphism and no evidence for eastward-directed lower crustal flow away from Tibet. We suggest that underthrusting of Indian lower crust north as far as the Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau.

Description: Die in der Stratigraphischen Tabelle von Deutschland 2016 (STD 2016) verwendete Zeitskala beruht mit kleinen Modifikationen auf den Komposit-Altern der Stratigraphischen Tabelle von Deutschland 2002 (STD 2002). Sie differiert von der ,,Geological Time Scale 2012" (GTS 2012). Die geochronologischen Einheiten zeigen auch die westeuropäischen Regionalstufen; die traditionelle deutsche Kulm-Gliederung wurde nicht mehr übernommen. Abfolgen des Mississippiums (unteres Karbon) sind in den deutschen Mittelgebirgen und im Untergrund Norddeutschlands weit verbreitet. Paralische Abfolgen des Pennsylvaniums (oberes Karbon) kommen übertage in der subvariszischen Saumsenke zwischen Aachen und Osnabrück sowie im Untergrund Norddeutschlands vor. Bis auf wenige Ausnahmen im Pennsylvanium einset- zende intramontane Abfolgen sind aus mehreren, zum Teil ausgedehnten Becken in Südwest- und Mitteldeutschland über- und untertage bekannt. Die vielgestaltige Fazies des deutschen Karbons wird von der nach Nordwesten progradierenden variszischen Orogenese und deren Ende im späten Westfalium gesteuert. Die Fazies reicht von Beckensedimenten und Flyschablagerungen sowie lateral anschließenden Plattformkalken im Mississippium (,,Kulm-Fazies" bzw. ,,Kohlenkalk- Fazies") zu paralischen und intramontanen, fast ausschließlich pennsylvanischen Molasse-Ablagerungen. Gegenüber der STD 2002 wurden die traditionellen Namen in den Standard-Profilen des Mississippiums von Aachen, Velbert und des Sauerlandes durch neue Formationsbezeichnungen ersetzt. Neu ist auch die Untergliederung des nordöstlichen und östlichen Rheinischen Schiefergebirges (Kellerwald, Lahn-Dill-Gebiet), wo durch Zirkon-Provenienz-Analysen rhenoherzynische und armorikanische Decken unterschieden wurden. Das Mississippium des Thüringisch-Vogtländischen Schiefergebirges und des Frankenwaldes wurde stärker differenziert und mit Formationsnamen belegt. Kleinere Änderungen gab es im Namurium der subvariszischen Saumsenke am NW-Rand des Rheinischen Schiefergebirges und in den intramontanen Pennsylvanium-Abfolgen der mitteldeutschen Becken. The Carboniferous timescale in the Stratigraphic Table of Germany 2016 (STD 2016) relies on slightly modified composite ages already used in the Stratigraphic Table of Germany 2002 (STD 2002). They differ from the Geological Time Scale 2012 (GTS 2012). Besides the international stages, the western European Mississippian regional stages are shown. The traditional German subdivision of the Kulm was discarded. Mississippian (lower Carboniferous) sedimentary successions are widespread in the mountainous regions and hills in the central part of Germany (Rhenish Mountains, Harz, Thuringian Forest, Franconian Forest and adjoining regions of Saxony). They are also widespread in the subsurface of Northern Germany. Paralic Pennsylvanian (upper Carboniferous) successions crop out in the Subvariscan Basin between Aachen and Osnabrück, and continue into the subsurface of Northern Germany. Intramontane successions, with few exceptions starting in the Pennsylvanian, occur in several, in part extended basins in southwestern and central Germany. They are known from outcrop and subsurface. The diversified facies of the Carboniferous in Germany is controlled by the northwestern progradation of the Variscan Orogeny and its finalisation during the late Westphalian. During the Mississippian, megafacies realms include deeper water basinal sediments and flysch deposits, and laterally adjoining shallow-water platform carbonates ("Kulm facies" and "Carboniferous Limestone facies", respectively). Locally starting in the later Mississippian (upper Viséan), paralic and purely continental intramontane molasse deposits prevail during the Pennsylvanian. The traditional lithostratigraphic terms of the Mississippian regional standard profiles of Aachen and the western and northern Rhenish Mountains (Velbert, Sauerland) were completely substituted by new formations. Lithostratigraphic terms of the northeastern and eastern Rhenish Mountains (Kellerwald, Lahn-Dill area) were completely revised, as provenance analyses of detrital zircons enabled the differentiation of Rhenohercynian and Armorican nappes. Also the Mississippian of the ThüringischFränkisches Schiefergebirge and the Frankenwald (Franconian Forest) are better differentiated; formations were introduced for most lithostratigraphic units. Minor modifications concern the Namurian of the Subvariscan Basin at the northwestern border of the Rhenish Mountains and the intramontane Pennsylvanian successions of several basins in central Germany.