NEODYMIUM AND STRONTIUM ISOTOPE STUDY OF THE BLUE-TIER BATHOLITH, NE TASMANIA, AND ITS BEARING ON THE ORIGIN OF TIN-BEARING ALKALI FELDSPARGRANITES

The middle to late Devonian plutons of the Blue Tier and Eddystone Bat holiths of NE Tasmania intrude quartz-wacke and mudstone of the Ordovi cian-Devonian Mathinna Beds. Four granitoid types are recognized: horn blende-biotite granodiorite, biotite granite, biotite-cordierite-garne t granite and alkali feldspar granite (AFG). Minor diorite bodies and dolerite dykes are also present. Tin (+W) mineralisation is geneticall y associated with the AFG. Major granitoid types (other than AFG) of t he Blue Tier and Eddystone Batholiths all have similar Nd-143/Nd-144 i nitial ratios (epsilon Nd = -5.0 to -6.5) at 370 Ma. More fractionated felsic samples within individual plutons have lower epsilon Nd values , suggesting progressive involvement (up to about 5%) of the sedimenta ry country rocks which have epsilon Nd Of about -11 at 370 Ma. The sou rce-rock model age for all granitoids (except AFG), based on a deplete d mantle evolution model (T-DM), is about 1.6 Ga. On the basis of geoc hemical data, Sr-87/Sr-86 initial ratios (0.707-0.713), oxygen isotope ratios (delta(18)0 = +9 to +12), and Rb/Sr and K/Ar isotopic age date s (368 to 388 Ma); the major plutons were interpreted to have been gen erated independently from heterogeneous crustal sources. Geochemical v ariation 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 systematic s of the Blue Tier Batholith of the Bassian Terrain are different from granite batholiths in the mainland of the Lachlan Fold Belt. Similari ty of isotopic systematics of the Blue Tier Batholith to those of Devo nian granites in central Victoria reinforces the geological correlatio n between western Tasmania and South Australia in the early Paleozoic. Two competing hypotheses for the generation of Sn-W mineralized AFG h ave been proposed on the basis of geological relationships, chemical a nd 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, throu gh processes of fractional crystallisation and subsequent metasomatic modification. The second hypothesis suggests that these AFG are unrela ted to the Poimena Granite; they are specialised granites derived from a different parent with distinctive chemical-isotopic systematics (Su n et al., 1986; Mackenzie, 1986; Mackenzie et al., 1988, 1990). Accord ing to the first hypothesis, the AFG exhibit a range in epsilon Nd ini tial values from -5.9 to -2.2. However, there is no known isotopic mec hanism which can produce this range within the geological time frame o f intrusion and solidification. At this time the authors favour the se cond hypothesis because of chemical and isotopic correlations. On the basis of a distinct discontinuity between Poimena and AFG on a Y versu s Ga/Al plot there is also a clear grouping of initial epsilon Nd for the Poimena Granite (around -6), and the AFG (-2.4). It is argued that those few ''AFG'' having much lower epsilon Nd than -2.4 are either s amples of altered Poimena Granite (two samples with epsilon Nd = -6.8 and -9.1) or a result of extensive metasomatic modification (one sampl e, epsilon Nd = -5.9) involving circulation of fluids in equilibrium w ith the country rocks (Poimena Granite initial epsilon Nd = -6 and Mat hinna Beds = -11). Hydrothermal minerals from the Anchor tin deposit s how a range in initial epsilon Nd and Sr-87/Sr-86 correlated with para genesis: apatite formed during albitisation of the Lottah Granite has Nd and Sr isotopic compositions intermediate between those of the Lott ah and Poimena Granites. Greisen fluorites have initial epsilon Nd bet ween -4.0 and -4.6, closer to the Lottah average composition, while in itial Sr-87/Sr-86 approaches that of the metasedimentary country rock (0.729). Fluorites formed during sericitic alteration have lower epsil on Nd compositions (< -6.1) and high Sr-87/Sr-86 (> 0.721). These data support an origin of tin mineralisation through mixing of an AFG-deri ved magmatic fluid with a externally-derived meteoric-groundwater domi nated fluid.