ON THE ANTARCTIC PENINSULA BATHOLITH

The plutonic rocks of the Antarctic Peninsula magmatic are form one of the major batholiths of the circum-Pacific rim. The Antarctic Peninsu la batholith is a 1350 km long by < 210 km wide structure which was em placed over the period similar to 240 to 10 Ma, with a Cretaceous peak of activity that started at 142 Ma and waned during the Late Cretaceo us. Early Jurassic and Late Jurassic-Early Cretaceous gaps in intrusiv e activity probably correspond to episodes of are compression. In a no rthern zone of the Antarctic Peninsula, the batholith intrudes Palaeoz oic-Mesozoic low-grade meta-sedimentary rocks, and in a central zone i t intrudes schists and ortho- and paragneisses which have Late Protero zoic Nd model ages and were deformed during Triassic to Early Jurassic compression. In a southern zone the oldest exposed rocks are Permian sedimentary rocks and deformed Jurassic volcanic and sedimentary rocks . All these pre-batholith rocks formed a belt of relatively immature c rust along the Gondwana margin. With few exceptions, Jurassic plutons crop out only within the central zone: many are peraluminous, having ' S-like' mineralogies and relatively high Sr-87/Sr-86(i). They are cons idered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion . By contrast, Early Cretaceous plutons crop out along the length of t he batholith. Few magma compositions appear to have been affected by u pper crust, the bulk being compositionally independent of the type of country rock they intrude. They are dominated by metaluminous, calcic, Si-oversaturated, I-type granitoid rocks with relatively low Sr-87/Sr -86(i), intermediate-silicic compositions (< 5 % MgO). We interpret th ese to represent partial melts of basic to intermediate, igneous, loca lly garnet-bearing, lower crust. Contemporaneous mafic magmas (e.g. sy n-plutonic dykes) form a more alkaline, Si-saturated series having hig her Nd-143/Nd-144 at the, Sr-87/Sr-86 than the intermediate-silicic se ries, to which they are not petro,goenetically related. The change fro m limited partial fusion of upper crust in Jurassic times to widesprea d partial fusion of lower crust in Early Cretaceous times is considere d to be a result of an increasing volume of basaltic intrusion into th e crust with time.