Abstract:

Some phengite xenocrysts were identified in early Cretaceous Zenong Group and Eocene Pana Formation volcanic rocks from Lhasa block. These white K-micas are generally subeuhedral to euhedral shape up to three millimeters large flakes with a reaction margin and are associated with sphene xenocryst. And they are not igneous minerals but xenocrysts. Electronic microprobe analysis suggests that contents of white K-micas vary from 3.29 to 3.49 Si atoms per formula unit, and 2.024 to 2.419 Al atom per formula unit, which belongs to typical high-pressure/ultrahigh pressure phengite. This discovery hints distinctively about high-pressure metamorphism or crustal thickening occurred in northern Lhasa block in early Cretaceous and southern Lhasa block in Eocene.

Key words: phengite, xenocryst in volcanic rocks, Lhasa block, crustal thickening