Lingtse gneiss (LGn) and Higher Himalayan crystallines (HHC) comprise parts of main central thrust (MCT) in the Darjeeling Sikkim Himalaya. Tourmaline bearing gneiss and quartz tourmaline veins are reported in immedia...Lingtse gneiss (LGn) and Higher Himalayan crystallines (HHC) comprise parts of main central thrust (MCT) in the Darjeeling Sikkim Himalaya. Tourmaline bearing gneiss and quartz tourmaline veins are reported in immediate contact with the LGn and some lesser Himalayan rocks in this study. Their importance is inferred via their comparative occurrence, micro-texture and chemistry. Flow of ductile crust was proposed to expose deep crustal rocks in the Himalayas in form of these gneissic rocks. Generation of paragneissic HHC from the protolith like the lesser Himalayan rocks like biotite-muscovite schist was proposed and documented in previous studies. The main central thrust where the principal motion is reported to date at circa 20 Ma appears in the contact regions of the HHC and lesser Himalayan rocks. Whether the tourmaline bearing gneiss or veins is a product during the episode of generation of the Higher Himalayan crystallines, which is taken as a component of the higher Himalayan crystallines episode remains a question as both concordant and discordant tourmaline bearing gneiss and/or quartzo-feldspahic veins appear respectively. The mm-cm scale tourmaline in the occasionally discordant quartz tourmaline veins shows strong zonation and less effects of shearing. Those are strongly zoned indicating magmatic hydrothermal character. The matrix tourmaline shows a separate composition. However, evidences of a less prominent shearing in them might signify rejuvenation. Lower temperature activity and fluid movement in this thrust zone are signified from the microstructure signifying that the high grade main central thrust was probably rejuvenated during or after the veining.展开更多
The Himalaya, a fold\|and\|thrust belt in the northern margin of the Indian continent, is characterized by thrust tectoncis (Schelling and Arita, 1991). It consists mainly of three thrust\|bounded lithotectonic units:...The Himalaya, a fold\|and\|thrust belt in the northern margin of the Indian continent, is characterized by thrust tectoncis (Schelling and Arita, 1991). It consists mainly of three thrust\|bounded lithotectonic units: from south to north the Sub\|Himalayan imbricate zone, the Lesser Himalayan thrust package (LH) and the Higher Himalayan thrust sheet (HH) with the overlying Tethys Himalayan sequence. These units are separated by a series of propagated thrusts, i.e. from south to north the Himalayan Frontal Fault (HFF), Main Boundary Thrust (MBT) and Main Central Thrust (MCT). These thrusts are splays off of an underlying mid\|crustal subhorizontal d$B;D(Jollement (Main Detachmen Trust or Main Himalayan Thrust), and were propagated southward with time. Among these thrusts the MCT is most important intracrustal thrust in considering the geological evolution of the Himalaya, and is controversial regarding its location and nature. In western and eastern Nepal the Higher Himalayan Crystalline sheet is thrust over the Lesser Himalayan rocks along the MCT. In the Kathmandu area of central Nepal also the high\|grade rocks of the HH with the overlying Tethyan sediments covers southward the Lesser Himalayan rocks, and form the Kathmandu nappe. In the north of the Kathmandu nappe the Higher Himalayan crystallines are skirted by the Main Central Thrust zone (MCT zone) which consists of green and black phyllites with sporadic garnet snow\|ball garnet and calcareous schist associated with characteristic mylonitic augen gneiss. The southern margin of the nappe is bounded by the Mahabharat Thrust (MT: Stoecklin, 1990) with a narrow zone of the LH which is cut by the MBT. But the relationship of the MCT in the north and the MT in the south is disputable and important (Arita et al., 1997: Rai et al., 1998: Upreti and Le Fort, 1999), and in the margin of the Kathmandu nappe the MCT zone has not been confirmed.展开更多
The Arun mega\|antiform, a large N—S structure transversal to the tectonic trend of the E Nepal Himalaya, is a tectonic window offering a complete section of the Himalayan nappe pile, from the Lesser Himalayan zone t...The Arun mega\|antiform, a large N—S structure transversal to the tectonic trend of the E Nepal Himalaya, is a tectonic window offering a complete section of the Himalayan nappe pile, from the Lesser Himalayan zone to the Tethyan Himalaya. At the northern end of the Arun tectonic window (ATW), the Ama Drime—Nyonno Ri range of south Tibet exposes a section of that portion of the Main Central Thrust (MCT) zone and Lesser Himalayan Crystallines (LHC) which elsewhere in Nepal is concealed below the overlying Higher Himalayan Crystalline (HHC) nappe (Fig. 1). As throughout the Himalaya at the structural level of the MCT, the ATW is characterized by an inverted metamorphic field gradient characterized by a progression from chlorite to sillimanite grade from low to high structural levels of the nappe pile. Metamorphic peak temperatures rise from circa 400℃ in the pelitic and psammitic Precambrian metasediments of the Lesser Himalayan Tumlingtar Unit, to 550～620℃ in the overlying LHC, to over 700℃ in the muscovite\|free Barun Gneiss, the lowermost HHC unit in the Arun valley.展开更多
The Arun Tectonic Window (ATW) and its inverted metamorphic zonation were first described by Bordet (1961) and Hagen (1969) in their regional surveys of the eastern Nepal Himalaya. The ATW is centred on the Arun antif...The Arun Tectonic Window (ATW) and its inverted metamorphic zonation were first described by Bordet (1961) and Hagen (1969) in their regional surveys of the eastern Nepal Himalaya. The ATW is centred on the Arun antiform (“ trans\|anticlinal de l’Arun”, Bordet, 1961), a major late structure, c. 100km long, which strikes north to north\|northeast, transversely to the E—W tectonic trend of the eastern Himalaya from the lower Arun valley to southern Tibet. From south to north, i.e. from the core of the window upwards in the nappe pile, the tectonic units exposed in the ATW are:(1) The Lesser Himalayan Tumlingtar Unit (Nawakot nappes of Hagen,1969), a thick sequence of greenschist\|facies Upper Precambrian metasediments, bounded to the north by a thrust zone (Main Central Thrust 1 of Maruo & Kizaki, 1983; Main Central Thrust Zone of Meyer & Hiltner, 1993). (2) The Lesser Himalayan Crystalline nappe (LHC), comprised of staurolite to kyanite grade micaschists and granitic orthogneiss (Kathmandu Nappes of Hagen,1969), lying on top of the low\|grade metasediments. (3) The Higher Himalayan Crystalline nappe (Tibetan Slab of Bordet, 1977), bounded on both side of the ATW by thrust sheets defining a major syn\|metamorphic thrust (Main Central Thrust of Bordet,1961; Main Central Thrust 2 of Maruo & Kizaki, 1983).In this contribution some results of geological investigations in the hitherto unrecognized northern part of the ATW (Kharta region of the Arun—Phung Chu valley and Ama Drime—Nyonno Ri range), are presented. The Kharta region is 30km east of the Everest—Makalu massif and sits in the western limb of the Arun antiform, whereas the Ama Drime—Nyonno Ri Range, to the east of Kharta, is right in the core of the Arun antiform. Here the ATW exposes a section of deep tectonic levels of the Lesser Himalayan Crystalline nappe and MCT zone which elsewhere in the Nepal Himalaya are concealed below the overlying Higher Himalayan Crystalline nappe.展开更多
文摘Lingtse gneiss (LGn) and Higher Himalayan crystallines (HHC) comprise parts of main central thrust (MCT) in the Darjeeling Sikkim Himalaya. Tourmaline bearing gneiss and quartz tourmaline veins are reported in immediate contact with the LGn and some lesser Himalayan rocks in this study. Their importance is inferred via their comparative occurrence, micro-texture and chemistry. Flow of ductile crust was proposed to expose deep crustal rocks in the Himalayas in form of these gneissic rocks. Generation of paragneissic HHC from the protolith like the lesser Himalayan rocks like biotite-muscovite schist was proposed and documented in previous studies. The main central thrust where the principal motion is reported to date at circa 20 Ma appears in the contact regions of the HHC and lesser Himalayan rocks. Whether the tourmaline bearing gneiss or veins is a product during the episode of generation of the Higher Himalayan crystallines, which is taken as a component of the higher Himalayan crystallines episode remains a question as both concordant and discordant tourmaline bearing gneiss and/or quartzo-feldspahic veins appear respectively. The mm-cm scale tourmaline in the occasionally discordant quartz tourmaline veins shows strong zonation and less effects of shearing. Those are strongly zoned indicating magmatic hydrothermal character. The matrix tourmaline shows a separate composition. However, evidences of a less prominent shearing in them might signify rejuvenation. Lower temperature activity and fluid movement in this thrust zone are signified from the microstructure signifying that the high grade main central thrust was probably rejuvenated during or after the veining.
文摘The Himalaya, a fold\|and\|thrust belt in the northern margin of the Indian continent, is characterized by thrust tectoncis (Schelling and Arita, 1991). It consists mainly of three thrust\|bounded lithotectonic units: from south to north the Sub\|Himalayan imbricate zone, the Lesser Himalayan thrust package (LH) and the Higher Himalayan thrust sheet (HH) with the overlying Tethys Himalayan sequence. These units are separated by a series of propagated thrusts, i.e. from south to north the Himalayan Frontal Fault (HFF), Main Boundary Thrust (MBT) and Main Central Thrust (MCT). These thrusts are splays off of an underlying mid\|crustal subhorizontal d$B;D(Jollement (Main Detachmen Trust or Main Himalayan Thrust), and were propagated southward with time. Among these thrusts the MCT is most important intracrustal thrust in considering the geological evolution of the Himalaya, and is controversial regarding its location and nature. In western and eastern Nepal the Higher Himalayan Crystalline sheet is thrust over the Lesser Himalayan rocks along the MCT. In the Kathmandu area of central Nepal also the high\|grade rocks of the HH with the overlying Tethyan sediments covers southward the Lesser Himalayan rocks, and form the Kathmandu nappe. In the north of the Kathmandu nappe the Higher Himalayan crystallines are skirted by the Main Central Thrust zone (MCT zone) which consists of green and black phyllites with sporadic garnet snow\|ball garnet and calcareous schist associated with characteristic mylonitic augen gneiss. The southern margin of the nappe is bounded by the Mahabharat Thrust (MT: Stoecklin, 1990) with a narrow zone of the LH which is cut by the MBT. But the relationship of the MCT in the north and the MT in the south is disputable and important (Arita et al., 1997: Rai et al., 1998: Upreti and Le Fort, 1999), and in the margin of the Kathmandu nappe the MCT zone has not been confirmed.
文摘The Arun mega\|antiform, a large N—S structure transversal to the tectonic trend of the E Nepal Himalaya, is a tectonic window offering a complete section of the Himalayan nappe pile, from the Lesser Himalayan zone to the Tethyan Himalaya. At the northern end of the Arun tectonic window (ATW), the Ama Drime—Nyonno Ri range of south Tibet exposes a section of that portion of the Main Central Thrust (MCT) zone and Lesser Himalayan Crystallines (LHC) which elsewhere in Nepal is concealed below the overlying Higher Himalayan Crystalline (HHC) nappe (Fig. 1). As throughout the Himalaya at the structural level of the MCT, the ATW is characterized by an inverted metamorphic field gradient characterized by a progression from chlorite to sillimanite grade from low to high structural levels of the nappe pile. Metamorphic peak temperatures rise from circa 400℃ in the pelitic and psammitic Precambrian metasediments of the Lesser Himalayan Tumlingtar Unit, to 550～620℃ in the overlying LHC, to over 700℃ in the muscovite\|free Barun Gneiss, the lowermost HHC unit in the Arun valley.
文摘The Arun Tectonic Window (ATW) and its inverted metamorphic zonation were first described by Bordet (1961) and Hagen (1969) in their regional surveys of the eastern Nepal Himalaya. The ATW is centred on the Arun antiform (“ trans\|anticlinal de l’Arun”, Bordet, 1961), a major late structure, c. 100km long, which strikes north to north\|northeast, transversely to the E—W tectonic trend of the eastern Himalaya from the lower Arun valley to southern Tibet. From south to north, i.e. from the core of the window upwards in the nappe pile, the tectonic units exposed in the ATW are:(1) The Lesser Himalayan Tumlingtar Unit (Nawakot nappes of Hagen,1969), a thick sequence of greenschist\|facies Upper Precambrian metasediments, bounded to the north by a thrust zone (Main Central Thrust 1 of Maruo & Kizaki, 1983; Main Central Thrust Zone of Meyer & Hiltner, 1993). (2) The Lesser Himalayan Crystalline nappe (LHC), comprised of staurolite to kyanite grade micaschists and granitic orthogneiss (Kathmandu Nappes of Hagen,1969), lying on top of the low\|grade metasediments. (3) The Higher Himalayan Crystalline nappe (Tibetan Slab of Bordet, 1977), bounded on both side of the ATW by thrust sheets defining a major syn\|metamorphic thrust (Main Central Thrust of Bordet,1961; Main Central Thrust 2 of Maruo & Kizaki, 1983).In this contribution some results of geological investigations in the hitherto unrecognized northern part of the ATW (Kharta region of the Arun—Phung Chu valley and Ama Drime—Nyonno Ri range), are presented. The Kharta region is 30km east of the Everest—Makalu massif and sits in the western limb of the Arun antiform, whereas the Ama Drime—Nyonno Ri Range, to the east of Kharta, is right in the core of the Arun antiform. Here the ATW exposes a section of deep tectonic levels of the Lesser Himalayan Crystalline nappe and MCT zone which elsewhere in the Nepal Himalaya are concealed below the overlying Higher Himalayan Crystalline nappe.
