Pumpellyite-bearing rocks in central Sweden and extent of host rock alteration as a control of pumpellyite composition

Metamorphic assemblages and mineral compositions in basic to intermediate volcanic rocks of two Precambrian units (the sub-Jotnian and Jotnian) in the Dala region, central Sweden, reveal a history comprising two metamorphic episodes. The sub-Jotnian rocks were metamorphosed during a first episode: the metamorphic grade ranges from prehnite-pumpellyite facies (in a displaced block with sub-Jotnian (?) lava occurring in the border zone of a major impact structure), to pumpellyite-actinolite facies, and to greenschist facies in the lower part of the unit. The unconformably overlying Jotnian rocks were metamorphosed to prehnite-pumpellyite facies during a subsequent episode, which also left its imprint in the sub-Jotnian basement. Two other Jotnian sequences in central Sweden are in the same facies. There are systematic chemical differences in some of the secondary minerals. Sphenes contain more Al, and epidotes and pumpellyites are more Fe-rich in Jotnian (prehnite-pumpellyite facies) rocks compared with sub-Jotnian rocks of pumpellyite-actinolite facies. Most epidotes in Jotnian lavas are remarkably rich in Fe, with a replacement of Al by Fe³⁺ up to 1.5 atoms per formula unit (Ps₅₀). Compositions of epidote and pumpellyite in pervasively altered rocks vary sympathetically. Pumpellyite composition is partly controlled by the extent of alteration in its host rock. There is a trend for the compositional field of pumpellyite to shrink and shift away from the Al corner of the Al-Fe*-Mg triangle with increasing extent of host rock alteration. This trend is most evident in Jotnian rocks. Since the fields in the Al-Fe*-Mg triangle given in the literature as representative of pumpellyites from different metamorphic fades are partly based on analyses of samples containing relicts, these fields should be adjusted if future investigations show that the trend is of general validity.