Anatexis Of Orthometamorphic Rock In Western Himalayan Orogen

Orthometamorphic rocks（gneiss,migmatite）are widely exposed in the Himalayan orogen,which is an important rock type of the Himalayan orogen.Whether orthometamorphic rocks are involved in crustal anatexis in Himalayan orogen is significant to the research of the petrogenesis of the Himalayan leucogranite.Leucogranite dykes produced in the orthometamorphic rocks may be the direct evidence to identify the orthometamorphic rock involved in crustal anatexis,and which completely records the information on the crustal material cycle,anatexis,and evolution of the tectonic background of the Himalayan orogen.In this paper,we used the orthometamorphic rocks and leucogranites that were exposed in the Southwest margin of Zhada Basin and the north slope of the Gurla Mandhata dome to be research objects.Through field geological survey,petrology,mineralogy,mineral geochemistry,petrochemistry,geochronology,and isotope geochemistry to determine the timing of anatexis of orthometamorphic rock,and discussed the mechanism of the anatexis of orthometamorphic rock,the petrogenesis of the Himalayan leucogranite,the source of the Himalayan leucogranite and the evolution of the tectonic background of the western Himalayan orogen.Through the study,we represent the following data and achievements.（1）Zhada granitic gneiss is composed mainly of quartz,plagioclase,biotite,and K-feldspar,the plagioclase exhibits a Carlsbad-albite compound twin.Zircon has an angular shape with an aspect ratio greater than 2.The cathodoluminescence images of zircons show that the core zircons of granitic gneiss have a clear and narrow oscillatory zoning structure.The augen granitic gneiss and migmatite in the Gurla Mandhata dome have an angular shape with an aspect ratio greater than 2.Cathodoluminescence images of zircons from the augen granitic gneiss and migmatite show clear and narrow oscillatory zoning at the rims of the zircons.The above mineralogical studies indicate that the protolith of granitic gneiss,augen granitic gneiss,and migmatites are magmatic rocks.（2）The zircon of the Zhada granitic gneiss has a core-rim structure.The core of the zircon shows a weighted ²⁰⁶Pb/²³⁸U age of 515±4 Ma,which indicates the protolith of the granitic gneiss was formed in the early Paleozoic;The rim of the zircon shows ²⁰⁶Pb/²³⁸U age of 17.9±0.5 Ma（Miocene）,which is similar to the weighted average age of the Zhada tourmaline granite（16.2±0.4 Ma）.The rim of zircon in granitic gneiss represents the time of the anatexis of the Zhada granitic gneiss.（3）The zircon of augen granitic gneiss have a core-mantle-rim structure,the inherited core of zircon from augen granitic gneiss contains the age of 838.0±15.0 to2146±53 Ma.The zircon rim of augen granitic gneiss contains a weighted average age of 483.7±4.8 Ma,which indicate the protolith of the augen granitic gneiss was formed in the early Paleozoic.Muscovite granite is produced in augen granitic gneiss in the Gurla Mandhata dome.Muscovite granite has a core-mantle-rim structure,the inherited core of zircon from muscovite granite contains the age of 685.0±13 to 2381±51 Ma.The inherited mantle of zircons from muscovite granite contains the weighted average age of 466.8±5.0 Ma and is similar to the weighted average age of zircon rim from augen granitic gneiss.It suggested that the orthometamorphic rocks represented by the augen granitic gneiss may be involved in crustal anatexis,and formed leucosome and a small amount of leucogranite dykes which include muscovite granite.The weighted average age of the zircon rim from Muscovite granite is16.0±0.2 Ma,which represents the time of anatexis of orthometamorphic rocks in the Gurla Mandhata dome.（4）The geochemical data of tourmaline granite samples collected from the Zhada area are similar to the geochemical characteristics of the muscovite dehydration partial melting.The highly variable Rb and Nb contents,Rb/Sr ratios,⁸⁷Sr/⁸⁶Sr（t）ratios（0.7445 to 0.8605）,andεNd（t）values（-3.6 to-8.2）indicate that the tourmaline granites are derived from a heterogenetic source.The general mixing equation calculated result of the Sr and Nd isotope shows that the heterogenetic source consisted of Tethyan Himalayan Sequence（THS）/Higher Himalayan Crystallines（HHC）metasediments and the granitic gneiss.The muscovite dehydration of the metasediments in the heterogenetic source produced fluid and triggered partial melting of the Zhada orthogneiss.（5）Petrographic studies show that the orthometamorphic rocks contain more than 20%biotite and muscovite,suggesting that biotite and muscovite may have no or little involvement in the crustal anatexis.The Sr-Nd isotopic data of Muscovite granite and garnet-bearing granite shows that ⁸⁷Sr/⁸⁶Sr（t）ratios andε_Nd（t）values have a negative correlation between leucogranite and orthometamorphic rocks in the Gurla Mandhata dome,which indicate that fluid may be involved in crustal anatexis.The rims of zircon from muscovite granite have relatively high crystallization temperatures（451-847℃）,which reaches the temperature of partial melting of the augen granitic gneiss,but fluid may also trigger partial melting of augen granitic gneiss.（6）Tourmaline granites may have been formed in the depressurized tectonic setting under the south-north extensional activity of the southern Tibet detachment system.Muscovite granites may be formed in the east-west tectonic extensional background of the orogenic belt.The weighted average age of muscovite granite（16.0±0.2 Ma）may represent the beginning time of the east-west extension in the western Himalayan orogen,and the tectonic transformation from the south-north extension to the east-west extension may take place at about 16 Ma.In conclusion,granitic gneiss,augen granitic gneiss,and migmatites which are exposed in the Zhada and the Gurla Mandhata area are the Orthometamorphic rocks.Orthometamorphic rocks in the western Himalayan orogen may be involved in the anatexis between 17.9 Ma to 16.0 Ma.The heterogenetic source of tourmaline granites may consist of Tethyan Himalayan Sequence（THS）/Higher Himalayan Crystallines（HHC）metasediments and the granitic gneiss.A number amount of leucogranites represented by Muscovite granites may be the products of low degree partial melting of orthometamorphic rocks during the crustal anatexis of the western Himalayan orogen,and inherited the geochemical characteristics of the orthometamorphic rocks.A small amount of leucogranite that partial melted from orthometamorphic rocks are not S-type granite in Western Himalayan orogen.Our research indicates that the protolith of metamorphic rocks is significant evidence to distinguish the genetic type of granite.