Felsic magmatism in the Darwin Glacier region, southern Victoria land, Antarctica

Simpson, Alanna Leigh.

A comprehensive study of the Darwin Glacier region has revealed at least four different styles of magmatism, creating the most diverse Cambrian magmatic region in southern Victoria Land. An alkaline suite, a calcic suite, an adakitic granitoid and a high-K dyke suite have been identified. The alkaline suite, the calcic suite and the adakitic granitoid were emplaced at mid-crustallevels (-16-21 km), compatible with the regional amphibolite facies metamorphic conditions of the country rock metasediments. Alkaline granitoids make up the Foggy Dog Granite (FDG) suite, which crops out at Pleasant Plateau and Cheney Bluff. The FDG suite is composed of moderately foliated, biotite-hornblende granites that show Fe20 3, Zr, Nb, Ga and Light Rare Earth Elements (LREE) (except Eu) enrichment and mineral chemistry (Mg# (100(Mg2+/Mg2++Fe2+)) less than 16) consistent with A-type granites. The suite is enriched in LREE relative to Heavy Rare Earth Elements (HREE) and shows significant negative Eu/Eu* anomalies (0.3-0.5). U-Pb isotopes indicate a crystallisation age of 536 ± 8 Ma. The FDG suite has isotopic and geochemical similarities to, and an emplacement age similar to, A-type granites found in the Koettlitz Glacier Alkaline Province (KGAP). The Darwin calcic suite is proposed to include the Cheney Bluff Granodiorite, the Blanks Granite and the Carlyon Granitoid. The Carlyon Granitoid makes up most of the Brown Hills and includes an variably foliated, biotite-hornblende granodiorite and granite. The Blanks Granite is a foliated, biotite granite pluton. The foliated, biotite Cheney Bluff Granodiorite intrudes the FDG suite at Cheney Bluff. The Darwin calcic suite post-dates the FDG suite and pre-dates the Cooper Granodiorite. The suite is a K-series, I-type, volcanic arc granitoid that is anomalously calcic and iron-rich compared to the DV1a suite and the Beardmore calcalkaline suite. The Darwin calcic suite is enriched in LILE (Rb, Ba, Sr, K, Th) and LREE (Ce, Nd) relative to N-MORB. Samples from the Carlyon Granitoid, the Blanks Granite and the Cheney Bluff Granodiorite all have negative Eu/Eu* anomalies (0.1- 0.3). The Cooper Granodiorite is an unfoliated, biotite-hornblende granodiorite that crops out north of the Brown Hills and west of Cheney Bluff. The Cooper Granodiorite is enriched in Sr and Na20 and depleted in Y and F~03 , relative to the Darwin calcic suite. Sr/Y ratios greater than 40 indicate an adakitic affinity. The Cooper Granodiorite has a similar geochemistry to the northern KGAP adakites and to the DVlb suite. The Cooper Granitoid has an re-interpreted U-Pb titanite crystallisation age of 515 Ma. There are two generations of felsic dykes in the Darwin Glacier region. The first generation is composed of highly fractionated dykes which intruded Darwin Glacier region plutons that were sufficiently hot, so chilled margins did not develop. The second generation is composed of high-K biotite aplites that intruded granitoids cool enough to induce rapid chilling. Both generations show similar chemistry to the FDG suite but are not enriched enough in Nb, Ce,Y, and Zr to be considered A-type granites. The second generation of aplites is similar to the DV2 suite of the Dry Valleys area. A highly oblique convergent margin formed in the late Proterozoic along the proto-Pacific margin of the East Antarctica Craton. This resulted in the formation of extensional jogs along the Transantarctic Mountains (TAMs), the largest of which produced the KGAP alkaline magmatism. Extension in the KGAP stretched the lithosphere, creating asthenospheric upwelling that melted the lower crust generating alkaline magmatism. Rotation of the interpreted plate vector initiated subduction first in the Darwin Glacier region, followed by the Dry Valleys region. The northern KGAP was isolated from subduction due to its orientation parallel to the relative plate motion. Approximately 15 Ma after subduction ceased, melting of either subducted lithosphere or a mafic underplate resulted in adakitic magmatism. Heterogeneous, evolved fractionates provided a source for the first generation of felsic dykes. Extension, uplift and partial melting of mantle and crusta} components resulted in the second generation of aplites.

xiv, 178, A60 p. : ill. (some col.), col. maps (2 folded) ; 30 cm.

2002Simpson_A

Abstract Only

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