TERTIARY BLOCK ROTATIONS AND PYRRHOTITE/ MAGNETITE GEOTHERMOMETRY IN THE TETHYAN HIMALAYA(SHIAR KHOLA,CENTRAL NEPAL)  

作　　者：E.Schill\+1,E.Appel\+1,P.Gautam\+2[WT5,5”BX](1.Institut für Geologie und Pal ontologie,Universitet Tübingen,Sigwartstrasse 10,72076 Tübingen, Germany 2.Central Department of Geology,Tribhuvan University,Kirtipur,Kathmandu,Nepal) 

出　　处：《地学前缘》2000年第S1期58-59,共2页Earth Science Frontiers

摘　　要：In Mesozoic carbonates of the Tethyan Himalayas two characteristic remanent magnetisations(ChRM\-1 and ChRM\-2)were identified by their unblocking spectra.The ChRM\-1 is carried by pyrrhotite(unblocking spectra:270～340℃),acquired as a secondary thermoremanent magnetisation (TRM) during exhumation and cooling.The ChRM\-2 is carried by magnetite (unblocking spectra:430～580℃).A primary origin is indicated by calcite twin geothermometry and remanences consistent with the expected direction.Along an E—W profile of 10km length the ratio of remanence intensity of pyrrhotite to magnetite ( R PYR/MAG )changes systematically (from 0 38 to 1 00,Fig.1).It is known that pyrrhotite is formed in marly carbonates during low\|grade metamorphism (Rochette 1987).This occurs at the expense of magnetite.Thus the ratio R PYR/MAG is related to metamorphic temperatures and can be used as a geothermometer for temperatures≤300℃ in low\|grade metamorphic carbonates where other methods are rare.Stable remanence directions were used to estimate the amount of block rotation around vertical and horizontal axes(i.e.Klootwijk et al.1985,Appel et al.1991 & 1995).In the Shiar area the pyrrhotite remanence directions follow a small\|circle distribution with a best fit parallel to the N—S direction(Fig.2).In Mesozoic carbonates of the Tethyan Himalayas two characteristic remanent magnetisations(ChRM\-1 and ChRM\-2)were identified by their unblocking spectra.The ChRM\-1 is carried by pyrrhotite(unblocking spectra:270～340℃),acquired as a secondary thermoremanent magnetisation (TRM) during exhumation and cooling.The ChRM\-2 is carried by magnetite (unblocking spectra:430～580℃).A primary origin is indicated by calcite twin geothermometry and remanences consistent with the expected direction.Along an E—W profile of 10km length the ratio of remanence intensity of pyrrhotite to magnetite ( R PYR/MAG )changes systematically (from 0 38 to 1 00,Fig.1).It is known that pyrrhotite is formed in marly carbonates during low\|grade metamorphism (Rochette 1987).This occurs at the expense of magnetite.Thus the ratio R PYR/MAG is related to metamorphic temperatures and can be used as a geothermometer for temperatures≤300℃ in low\|grade metamorphic carbonates where other methods are rare.Stable remanence directions were used to estimate the amount of block rotation around vertical and horizontal axes(i.e.Klootwijk et al.1985,Appel et al.1991 & 1995).In the Shiar area the pyrrhotite remanence directions follow a small\|circle distribution with a best fit parallel to the N—S direction(Fig.2).

关 键 词：PALAEOMAGNETISM TETHYAN HIMALAYAS block rotation geoghermormetry 

分 类 号：P5[天文地球—地质学]