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The Proterozoic Y terrane of the Reading Prong of eastern Pennsylvania and New Jersey consists of light-colored, sodic-rich rocks containing intercalated amphibolite, the Losee Metamorphic Suite, calcarerous and quartzofeldspathic metasedimentary rocks, the intrusive Hexenkopf Complex, the Byram Intrusive Suite, and quartz-poor monzonite, syenite, and related pyroxene granite.

The Losee consists of oligoclase-quartz gneiss and amphibolite that in places has been partly mobilized to form venite and albite oligoclase granite. Rocks of charnockitic affinity may be a partial melt of an amphbiolite-rich phase of the Losee. The Losee is thought to be metamorphosed quartz keratophyre and related sodic basalt. It is probably basement to the calcareous and quartzofeldspathic metasedimentary rocks.

The calcareous rocks are mostly marble, amphibolite, pyroxene gneiss, and epidote- and scapolite-bearing gneisses. They are interlayered with quartzofeldspathic gneiss of two general types: biotite-quartz-feldspar gneiss and potassic feldspar gneiss. At places, the potassic feldspar gneiss has melted and has formed small bodies and layers of potassium-rich granite. The quartzofeldspathic rocks are of continental margin type and are thought to be a clastic wedge containing layers and lenses of calcareous rocks. The source of the clastic material was probably a granitic terrane because of the large amount of potassic feldspar. Some volcanic material may be present in this sequence, but the evidence is equivocal.

The Hexenkopf Complex consists of severely altered mafic plutonic rock. It appears to lie beneath the Losee, and if so, is the oldest known rock in this part of the Reading Prong.

The Byram Intrusive Suite consists of hornblende granite and alaskite that form syntectonic and conformable sheets within the metamorphic rocks, as well as scattered small bodies of biotite granite that resulted from the granitization of biotite-quartz-feldspar gneiss. Byram leucosome forms arterites from both biotite-quartz feldspar gneiss and amphibolite. The Byram probably results from the anatectic melting of older rocks, but at the present level of erosion, there is no evidence that this has taken place. The Byram probably had its origin in the source terrane of the quartzofeldspathic gneiss.

The quartz-poor monzonite, syenite, and related pyroxene granite are not well understood. These rocks form syntectonic conformable sheets like the rocks of the Byram Intrusive Suite, with one exception, in which a sheet of quartz syenite appears to cut across the structure of hornblende granite. These rocks are also probably anatectic, perhaps originating at a lower level than the Byram. This place of origin is not certain, however, because the relation of the quartz-poor rocks to the Byram is not really known.

The rocks in eastern Pennsylvania and New Jersey were metamorphosed in at least upper amphibolite facies, and most were probably metamorphosed in hornblende granulite facies. There is some evidence of polymetamorphism in northeasternmost New Jersey, but that concept needs further evaluation.

The metamorphic, intrusive, and deformational event in the Reading Prong can be dated at about 1 b.y. ago, so it is clearly Grenvillian. The rocks in the Reading Prong are very much like those of the Adirondacks and probably like those of the Honey Brook Upland. They have similarities to rocks of the Green Mountains, Berkshires, and basement massifs of western Connecticut. They are not at all like the rocks of Avondale-West Chester Massif, the Baltimore Gneiss, or the rocks of the northern and southern Blue Ridge.

In a few small areas, the Proterozoic Y rocks are overlain by a sequence of interlayered metasedimentary and metavolcanic rocks that are named the Chestnut Hill Formation. These rocks are at a lower metamorphic grade and are much less homogenized than the Proterozoic Y rocks and are thought to be of probable Proterozoic Z age.

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