TERTIARY BLOCK ROTATIONS AND PYRRHOTITE/ MAGNETITE GEOTHERMOMETRY IN THE TETHYAN HIMALAYA(SHIAR KHOLA,CENTRAL NEPAL)
TERTIARY BLOCK ROTATIONS AND PYRRHOTITE/ MAGNETITE GEOTHERMOMETRY IN THE TETHYAN HIMALAYA(SHIAR KHOLA,CENTRAL NEPAL)
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).
出处
《地学前缘》
EI
CAS
CSCD
2000年第S1期58-59,共2页
Earth Science Frontiers
参考文献6
-
1Appel E,Patzelt A,Chouker C.Secondary palaeoremanence of Tethyan sediments from the Zanskar Range (NW Himalaya)[].Geophysical Journal International.1995
-
2Besse J,Courtillot V.Revised and Synthetic Apparent Polar Wander Paths of the African Eurasian, North American and Indian Plates, and True Polar Wander Since 200 Ma[].Journal of Geophysical Research.1991
-
3Guillot S,Hodges K,LeFort P,et al.New constraints on the age of the Manaslu leucogranite: Evidence for episodic tectonic denudation in the central Himalayas[].The Journal of Geology.1984
-
4Klootwijk C,T,Conaghan P J,Powell C McA.The Himalayan arc: Large scale continental subduction, oroclinal bending and back-arc spreading[].Earth and Planetary Science.1985
-
5Rochette P.Metamorphic control of the magnetic mineralogy of black shales in the Swiss Alps: toward the use of magnetic isogrades[].Earth and Planetary Science.1987
-
6Wyss M,Hermann J,Steck A.Structural and metamorphic evolution of the northern Himachal Himalaya, NW India[].Eclogae Geologicae Helvetiae.1999