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Field, petrological and geochemical study of these tin granites shows that they form composite and epizonal plutons containing several textural facies, including medium- to coarse-grained seriate, porphyritic with aplitic groundmass, and equi- to sub-equigranular biotite granites. These granites formed during the Late Cretaceous, following more voluminous granodiorite and granite emplacement at greater depths. This relationship, as well as their overall geochemistry and high initial 87Sr/86Sr ratios (0.708-0.720), suggest that the granites owe their origin primarily to fractional crystallization of much larger crust-derived parent batholiths at depth. However, the composite stocks of the tin granites underwent their own evolution to produce a small volume of late-stage, geochemically very evolved residual magma. The evolution included the presence of a volatile phase during the final stages and its subsequent depletion. Despite their having some characteristics of the S-type granites, I- and S-type criteria for granite genesis are not applicable to the tin granites.-L.H.