HR: 0830h
AN: T21D-0479    [PDF]
TI: Tectono-Metamorphic Events Along the Dangjin Pass Transect of the Altun Shan, Western China: Conditions, Timing and Tectonic Implications
AU: * Genser, J
EM: johann.genser@sbg.ac.at
AF: Institute of Geology, University Salzburg Hellbrunnerstr. 34, Salzburg, 5020 Austria
AU: Liu, Y
EM: liuyj@email.jlu.edu.cn
AF: Faculty of Earth Sciences, Jilin University, Changchun, 130061 China
AU: Neubauer, F
EM: franz.neubauer@sbg.ac.at
AF: Institute of Geology, University Salzburg Hellbrunnerstr. 34, Salzburg, 5020 Austria
AU: Friedl, G
EM: gertrude.friedl@sbg.ac.at
AF: Institute of Geology, University Salzburg Hellbrunnerstr. 34, Salzburg, 5020 Austria
AB: The Altun Shan range harbors the Altyn fault that shows a cumulative post-Jurassic sinistral offset of ca. 400 km. Aside from this young tectonics, little is known about the evolution of this boundary structure of the Tibetan plateau. We studied the Dangjin pass transect, where two major shear zones can be discerned, a northern brittle fault, which delimits metamorphic rocks of the Altun Shan to alluvial sediments in the north and a southern ductile fault zone characterized by low-grade, retrograde metamorphic rocks. To the south, again medium grade metamorphic rocks crop out. The northern zone displays an increase in metamorphic grade from the biotite zone to staurolite-in. Average PT calculations gave 6 kbar and $550\deg$C (garnet zone) to $615\deg$C. The ductile shear zone displays lower greenschist metamorphic conditions with relics of higher grade minerals. The southern unit shows a two-stage metamorphic evolution, recorded by distinct mineral compositions. PT conditions increase from $550\deg$C/4 kbar to $535\deg$C/7.5 kbar during the first event, and are ca. $620\deg$C/7 kbar for the overprint. Along the transect, four Ar/Ar age groups are evident: (1) ages of ca. 450 Ma between the two shear zones; (2) ages of ca. 165 Ma in rocks bordering the ductile shear zone; (3) ages down to ca. 30 Ma within the latter; and (4) ages of ca. 350 Ma in metamorphic rocks of the southern zone. Muscovite from the southern shear zone gave a total gas age of 32.5 Ma, K-feldspar shows a stair-case pattern with ages from ca. 32 Ma to 137.5 Ma. Hornblende from the southern unit yielded total gas ages of 395 and 425 Ma. These data allow to discern four distinct tectono-metamorphic events. Ordovician (about 450 Ma) metamorphism and cooling, which signifies a major regional event. A second, late Devonian-early Carboniferous (about 350 Ma) regional event occurred south of Dangjin pass, that overprinted parageneses of the first phase. Within the Altun Shan, a restricted mid-Jurassic (ca. 165 Ma) event is observable, and finally strong retrograde metamorphism and shearing at about 30 Ma, which denotes a ductile precursor stage to the active Altyn Tagh fault.
DE: 1035 Geochronology
DE: 3660 Metamorphic petrology
DE: 8102 Continental contractional orogenic belts
SC: Tectonophysics [T]
MN: 2003 Fall Meeting