Cordierite reaction textures in semipelitic rocks from metamorphic zone IV in central Massachusetts: Their interpretation and possible p-t significance
- Mineralogisch-Petrographisches Inst., Freiburg (Germany)
- Univ. of Massachusetts, Amherst, MA (United States). Dept. of Geology and Geography
In central Massachusetts, examples of cordierite (CRD)-garnet (GAR) assemblages are rare outside of the Acadian metamorphic Zone VI (lower granulite facies). The previously known exceptions are limited to some gedrite-bearing assemblages within a distinct stratigraphic unit from Zone 2. Consequently, the discovery of CRD + GAR in a thin horizon within layered felsic and mafic gneisses from Zone IV was somewhat surprising. This horizon contains CRD, GAR, pale-green biotite (BIO), brown BIO, sillimanite (SIL), staurolite (ST), plagioclase (PLG) and quartz (QZ). In thin section, CRD has two modes of occurrence: (i) CRD blasts enclose fine mats of SIL needles that are woven through pale-green BIO flakes, CRD and fine-grained QZ and (ii) as partial coronas along with brown BIO around GAR. Outside of the CRD, SIL is scarce and is found with rare ST and brown BIO. These textures, the presence of two types of BIO and regional P-T conditions suggest the following scenario: (1) muscovite (MUS) + GAR + QZ reacted to form brown BIO + CRD, which (2) resulted in partial enclosure of GAR by BIO and CRD and entrapment of MUS in CRD; subsequently, MUS + CRD reacted to form pale-green BIO + SIL + QZ, which resulted in the final destruction of MUS. The regional occurrence of SIL pseudomorphs after andalusite (AND), peak Zone IV conditions of about 670 C and 6 kbar, estimates of about 600 C and 5 kbar for the brown BIO + CRD + GAR assemblage, and the approximate P-T orientations of the reactions discussed above suggest that the P-T path for zone IV was generally clockwise. Evolution of the P-T path may have been as follows: the ANDSIL transition marked the earliest point on the P-T path; productions of CRD (above) probably indicates decompression followed peak metamorphic conditions; and the still later growth of pale-green BIO + SIL suggests cooling in the SIL stability field.
- OSTI ID:
- 5587942
- Report Number(s):
- CONF-9303211-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:2; Conference: 28. annual Geological Society of America (GSA) Northeastern Section meeting, Burlington, VT (United States), 22-24 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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