عنوان

Characterization and timing of metamorphism within the Indo-Asian suture zone, Himalayas, northern Pakistan

TLpq304041019

انگلیسی

Characterization and timing of metamorphism within the Indo-Asian suture zone, Himalayas, northern Pakistan

[Thesis]

H. A. Smith

C. P. Chamberlain

Dartmouth College

1993

304

Ph.D.

Dartmouth College

1993

The Himalayas of northwestern Pakistan mark the zone of deformation and metamorphism related to the collision of India northward into Asia. Geologic mapping in one region where the boundary between the Indian and Asian plates is well-exposed is the basis for a detailed look at the nature and timing of Himalayan metamorphism. In the Upper Kaghan Valley, Indian plate rocks have been overthrust by the Kohistan Island Arc, previously accreted to Asia, along the Main Mantle Thrust (MMT). Immediately below the MMT appear a band of graphitic metapelites interpreted to relate directly to the thrusting. These rocks show metamorphism to mid-amphibolite facies conditions, with garnet-biotite geothermometry indicating temperatures of ca. 500C. Some of the mineral assemblages from these rocks contain chloritoid as a metastably preserved relict mineral. Such occurrences offer the opportunity to follow the metamorphic trajectory of the rocks using the reaction (modal) space approach of Thompson (1982b). At structurally deeper levels within the Indian plate, metamorphic conditions are additionally constrained to 500-550C at pressures of 7-8 kbar. Syntectonic granites intruding these rocks give zircon U-Pb ages of ca. 45-50 Ma. As these ages date thermal culmination of metamorphism related to the Indo-Asian collision, they raise questions concerning the timing of the onset of Himalayan orogeny. The duration of the orogeny is also uncertain, as direct dating of peak metamorphism, using monazite from metamorphic rocks of the Nanga Parbat-Haramosh Massif (NPHM) on the Indian plate and from the Karakorum region of the Asian plate, give U-Pb ages of between 4 and 11 Ma. The results from the NPHM corroborate independent evidence of quite recent metamorphic activity. However, the young ages from the Karakorum are wholly surprising. Calculations modeling the diffusive Pb loss in monazite resulting from tectonic reduction in crystal size suggest this is not a mechanism to produce the enigmatic U-Pb results.

Earth sciences

Geology

Geology

Mineralogy

C. P. Chamberlain

H. A. Smith

p

[Thesis]

276903

a

Y