Magnetomineralogical changes along the Kohistan-Karakoram collision zone in North Pakistan: implications for variable thermochemical activities

Authors: HAIDER ZAMAN, ABDULLAH O BAMOUSA

Abstract: To identify remanence-carrying magnetic minerals and document their thermochemical behavior, detailed rock magnetic investigations were conducted on Cretaceous-Tertiary rocks collected from northwestern and northeastern Kohistan, northern Pakistan. Rock magnetic results from each area are clearly differentiated on the basis of site location. Detrital hematite is identified as a main magnetic carrier in red beds of northwestern Kohistan located at some distance from the Northern Suture Zone with subsidiary pigmentary hematite and altered magnetite. In contrast, the magnetic mineralogy of purple-colored rocks adjacent to the Northern Suture Zone is shared by unstable secondary pyrrhotite, magnetite, and hematite. The magnetic mineralogy of northeastern Kohistan is dominated by recently formed pyrrhotite with contributions from secondary hematite and magnetite. The widespread presence of pyrrhotite in the eastern sector indicates contrasting thermochemical regimes. Two geological implications are inferred from magnetization contrasts in the eastern and western sectors of the Kohistan Range. First, rocks at a distance from the Northern Suture Zone are less intensely remagnetized than those closer to it and provide a signature of the thermochemical gradient. Second, rocks in northeastern Kohistan are more intensely remagnetized compared to their counterparts in the northwest, implying an increase in the thermochemical gradient from west to east. The reported occurrences of numerous sulfide mineral zones in the study areas support our interpretation of intense remagnetization as a result of increased thermochemical activity near the Northern Suture Zone.

Keywords: Kohistan, Northern Suture Zone, magnetic mineralogy, red beds, detrital hematite, pyrrhotite, remagnetization

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