Early Jurassic felsic and associated mafic meta-igneous rocks in Otago Schist, Central Otago, New Zealand

摘要

Metamorphosed felsic volcanic rocks occur interlayered with metasedimentary schist in close association with a 7km long metabasaltic (greenschist) horizon in the Otago Schist belt of Central Otago. The felsic metavolcanic rocks are weakly foliated and locally segregated and consist principally of quartz and albite. Geochemical analyses suggest phonolitic compositions and a general within-plate affinity for the felsic metavolcanics rocks, and a MORB/back arc basalt affinity for the associated metabasalts. Zircons from two felsic metavolcanic samples are stubby, sharply faceted, prismatic grains with no visible inherited cores, and are considered to be of primary magmatic origin. Calculated weighted average Pb-206-U-238 ages for zircons from the two felsic metavolcanic samples are 190.5 +/- 0.6Ma and 190.1 +/- 0.3Ma. We interpret these ages (late Aratauran-early Ururoan; middle Early Jurassic) to date the deposition of the Torlesse Terrane metasediments at that locality, and also possibly the emplacement of the associated metabasalts.