Neodymium and strontium isotope study of the Blue Tier Batholith, NE Tasmania, and its bearing on the origin of tin-bearing alkali feldspar granites

Abstract

The middle to late Devonian plutons of the Blue Tier and Eddystone Batholiths of NE Tasmania intrude quartz-wacke and mudstone of the Ordovician-Devonian Mathinna Beds. Four granitoid types are recognized: hornblende-biotite granodiorite, biotite granite, biotite-cordierite-garnet granite and alkali feldspar granite (AFG). Minor diorite bodies and dolerite dykes are also present. Tin (+ W) mineralisation is genetically associated with the AFG. Major granitoid types (other than AFG) of the Blue Tier and Eddystone Batholiths all have similar 143Nd/ 144Nd initial ratios (ϵNd = − 5.0 to − 6.5) at Ma. More fractionated felsic samples within individual plutons have lower ϵNd values, suggesting progressive involvement (up to about 5%) of the sedimentary country rocks which have ϵNd of about − 11 at 370 Ma. The source-rock model age for all granitoids (except AFG), based on a depleted mantle evolution model (T DM), is about 1.6 Ga. On the basis of geochemical data, 87Sr/ 86Sr initial ratios (0.707-0.713), oxygen isotope ratios (δ 180 = + 9 to + 12), and Rb/Sr and K/Ar isotopic age dates (368 to 388 Ma), the major plutons were interpreted to have been generated independently from heterogeneous crustal sources. Geochemical variation within each major granitoid type, with the possible exception of the AFG, is due to fractional crystallisation accompanied by some country rock assimilation. Integrated Nd, Sr and O isotopic systematics of the Blue Tier Batholith of the Bassian Terrain are different from granite batholiths in the mainland of the Lachlan Fold Belt. Similarity of isotopic systematics of the Blue Tier Batholith to those of Devonian granites in central Victoria reinforces the geological correlation between western Tasmania and South Australia in the early Paleozoic. Two competing hypotheses for the generation of SnW mineralized AFG have been proposed on the basis of geological relationships, chemical and mineralogical trends and stable and radiogenic isotopes. The first hypothesis (Groves et al., 1977; Higgins et al., 1985; Higgins, 1990) links the AFG with the country rock, more mafic Poimena Granite, through processes of fractional crystallisation and subsequent metasomatic modification. The second hypothesis suggests that these AFG are unrelated to the Poimena Granite; they are specialised granites, derived from a different parent with distinctive chemical-isotopic systematics (Sun et al., 1986; Mackenzie, 1986; Mackenzie et al., 1988, 1990). According to the first hypothesis, the AFG exhibit a range in ϵNd initial values from − 5.9 to − 2.2. However, there is no known isotopic mechanism which can produce this range within the geological time frame of intrusion and solidification. At this time the authors favour the second hypothesis because of chemical and isotopic correlations. On the basis of a distinct discontinuity between Poimena and AFG on a Y versus Ga/Al plot there is also a clear grouping of initial ϵNd for the Poimena Granite (around −6), and the AFG (− 2.4). It is argued that those few “AFG” having much lower ϵNd than − 2.4 are either samples of altered Poimena Granite (two samples with ϵNd = − 6.8 and − 9.1) or a result of extensive metasomatic modification (one sample, ϵNd= − 5.9) involving circulation of fluids in equilibrium with the country rocks (Poimena Granite initial ϵNd = − 6 and Mathinna Beds = − 11). Hydrothermal minerals from the Anchor tin deposit show a range in initial ϵNd and 87Sr/ 86Sr correlated with paragenesis: apatite formed during albitisation of the Lottah Granite has Nd and Sr isotopic compositions intermediate between those of the Lottah and Poimena Granites. Greisen fluorites have initial ϵNd between − 4.0 and − 4.6, closer to the Lottah average composition, while initial 87Sr/ 86Sr approaches that of the metasedimentary country rock (0.729). Fluorites formed during sericitic alteration have lower ϵNd compositions (< −6.1) and high 87Sr/ 86Sr (> 0.721). These data support an origin of tin mineralisation through mixing of an AFG-derived magmatic fluid with a externally-derived meteoric-groundwater dominated fluid.