A Late Episode of Ferrar Magmatism? Geochemistry and Preliminary 40Ar/39Ar Results of High Fe (SPCT) Tholeiites from Southern Victoria Land, Antarctica

Abstract

The Middle Jurassic Ferrar Magmatic Province extends along the proto-Pacific margin of Gondwana from Antarctica to Tasmania, Australia and New Zealand. This magmatism is part of the Mesozoic low-Ti Gondwana Province (Cox, 1988), but differs from the other Mesozoic Large Igneous Provinces, related to the break-up of Gondwana, by having high SiO2 content, LILE/HFSE ratios and Sr isotopic compositions (87Sr/86Sr > 0.708). In Antarctica, the Ferrar magmatism is represented by the mafic layered intrusions of Dufek, the sills and dikes of the Ferrar Dolerite and by the lava flows of the Kirkpatrick Basalt. Geochemical and isotopic studies on the Kirkpatrick basalts from the Mesa Range (Victoria Land) revealed the occurrence of two geochemical types: the Mount Fazio (low-Fe; MFCT) and the overlying Scarab Peak (high-Fe; SPCT; Fleming et al., 1995). Ferrar sills and lavas from Prince Albert Mountains have been investigated for major and trace elements and Sr-Nd isotopic compositions. The studied rocks mainly belong to the low-Fe tholeiites (MFCT) and consist of tholeiitic-andesitic basalts and andesites, while the high-Fe tholeiites (SPCT) have been found only at Brimstone Peak. High-Fe (SPCT) rocks are represented by evolved (rag# 23-18) andesites characterized by relatively high FeOt (>I3.0 wt.%) and TiO2 (>1.8 wt.%) and low SiO2 (56-59) contents. Relative to the high-Fe (SPCT) rocks, the low-Fe (MFCT) rocks cover a wider range of mg# (54-23) showing lower FeOt (<12.0 wt.%) and TiO2 (<1.4 wt.%) and, for similar rag# values, higher SiO2 (56-61) contents. The ~Nd and aSr values of the low-Fe (MFCT) tholeiites range from -4.8 to -5.7 and from 85.8 to 112.1, respectively. The ~Sr values are positively correlated with the degree of differentiation, (e.g. increase of SiO2 content and L R E E / H R E E ratios), suggesting that crustal contamination may operated during the magmatic differentiation of low-Fe (MFCT) tholeiites. The high-Fe (SPCT) rocks exhibit less em'iched Sr-Nd isotopic compositions (~Nd = -2.7 to -3.9; 8Sr =73.6 to 78.9), with respect to the high-Fe (MFCT) indicating that the SPCT rocks can not derive from the MFCT magmas by fractional crystallization. This suggests SPCT and MFCT experienced different evolutionary processes, and may require distinct parental magmas. Previous 4~ data suggested that Kirkpatrick basalts and Ferrar dolerites were emplaced in a short time interval, less than 1 Ma (e.g. Heimann et al., 1994; Fleming et al., 1997). We present new 4~ data on an high-Fe (SPCT) andesite. The plagioclase separate of this sample yields an 4~ plateau age of 175.1 _ 1.0 Ma (lcy analytical precision; Fig. 1). This plateau is defined by 21 contiguous steps and by c. 90% of released 39Ar. The isochron age (174.9 + 2.5 Ma; MSWD = 0.95; intercept 4~ =