The Honey Brook Upland: Multiple Accessory Phase Parageneses in a Grenville Terrane and Associated Cover Sequence
Abstract
The Honey Brook Upland (HBU) of southeastern Pennsylvania is the only Grenville-age AMCG suite exposed between the Adirondacks and the anorthosite-bearing terranes of central and eastern Virginia. Several distinct accessory phase parageneses in the HBU and its Paleozoic metasedimentary cover sequence help: 1) to constrain the timing of known events affecting the HBU and cover; 2) to identify previously unknown events, and; 3) to elucidate the T-x conditions of the distinct metamorphic events. Granulite-facies gneisses (charnockites, mangerites) associated with the Honey Brook anorthosite contain primary Zrn, Aln, and Ap, but also texturally late Mnz in Bt+Hbl coronas around Opx, and (with Xno) as oriented acicular inclusions in primary Ap. The latter texture is interpreted as evidence of metasomatic infiltration (Harlov et al., Am Min, 2002), and Mnz-Xno pairs in Ap yield temperatures of 450° C-500° C. Amphibolite-facies felsic gneisses (metavolcanics) contain primary Zrn, Mnz, and Ap; Mnz rims are commonly replaced by Aln+Ep. Mnz in the metavolcanics has three distinct compositional domains; moderate-Y, low-Th cores, low-Y, high-Th outboards, and rare low-Y, low-Th rims. Garnets in the metavolcanics are only slightly zoned in Y (1000-800 ppm core to rim); Grt rims are typically replaced by Ep+Ms. Pairing of Grt compositions and Mnz core domains yields temperatures of 500° C-520° C, indicating preservation of prograde Mnz. The association of Qtz+Plg+Ms symplectites with high-Th Mnz domains suggests that this domain is representative of peak metamorphic conditions (650° C-750° C). Coexisting Mnz and Xno are found in metaquartzite (Chickies Formation) at the base of the HBU cover sequence; epitaxial overgrowths of Xno on oscillatory-zoned Zrn imply anin situ origin for Xno. Mnz-Xno pairs in the metaquartzite yield equilibration temperatures of 300° C-400° C. A provisional sequence of events in the HBU and cover, as determined from chemical Mnz ages, is as follows: 1) intrusion of anorthosite and charnockite, plus eruption of calc-alkaline volcanics, prior to 1070 Ma; 2) metamorphism of the HBU between 1050 and 950 Ma; Mnz cores in metavolcanics are dated at 1000±48 Ma, and high-Th Mnz averages 973±64 Ma; 3) metasomatic infiltration of HBU; late Mnz in Bt+Hbl coronas forms at 703±38 Ma, possibly coeval with deposition of the Chickies Formation; 4) overprint of HBU and greenschist-facies metamorphism of the cover sequence. Imprecise ages of Mnz in metaquarzite (235±50 Ma) imply that the greenschist-facies event is either late Paleozoic or early Mesozoic. Acadian (sl) ages, which are common in the Wissahickon schist south of the HBU cover sequence, have not as yet been identified in the HBU cover sequence.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.V33A..04P
- Keywords:
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- 3625 Descriptive mineralogy;
- 3660 Metamorphic petrology;
- 3670 Minor and trace element composition;
- 3694 Instruments and techniques