ZR-RICH, TH-RICH, AND REE-RICH BIOTITE DIFFERENTIATES IN THE A-TYPE GRANITE PLUTON OF SUZHOU (EASTERN CHINA) - THE KEY ROLE OF FLUORINE

The Suzhou Cretaceous (Yanshanian) pluton is a small high-level anorog enic composite body of two different biotite granite intrusions. They correspond to two batches (with an apparent gap of 10 Ma) of magma wit h A-type affinities. Sub-, trans- and hypersolvus textures of feldspar s are present in the first intrusion, whereas hypersolvus textures dom inate in the second. Biotite appears as a late (interstitial or in clu ster) magmatic phase. All the granites are Si-alkali-F rich and Mg-Ca- Ti poor, and can be termed highly fractionated felsic granites. HFSE ( but also some LILE) content is high and increases dramatically with in ferred differentiation. If a classical fractional crystallization mode l can be invoked for the first intrusion, a more complicated petrogene tic path has be considered for the second. Biotite-rich sequences or l ayers up to true 'biotite' bodies (up to 30 vol.% of F-rich, annite-li ke biotite) occur structurally close to the roof of the second intrusi on. Together with their high biotite content, these bodies are charact erized by a particularly rich accessory suite, where zircon, a Th-, Ca -rich fluoride, and fluorite are dominant, which is responsible for hi gh chemical anomalies (3.5 wt.% Zr, 0.52 wt.% Th, 5.42 wt.% F in whole -rock) and for a strong asymmetrical partitioning of HREE [(La/Yb)N << 1] in the biotite-rich bodies. A petrogenetic model of a water-poor, F-rich, high-temperature magma which becomes volatile saturated late i n the crystallization sequence (with biotite as an interstitial phase, and miarolitic cavities), in a subvolcanic setting (porphyritic and g ranophyric textures are present) seems likely. The large increase in m ost of the HFSE correlates with F. This correlation originated at the magmatic stage. Subsequent fluorine complexing is assumed to have scou red and transported these HFSE as soluble components. The 'biotitite' differentiates are assumed to result from the reaction of these F-rich fluids with the surrounding granite along specific structural traps c lose to the roof of the second intrusion. Zr, Th, and REE enrichment i n residual melt appeared as a consequence of the initial alkaline char acter of the melt and was followed by alkali loss through degassing. I sotopic constraints on the source of magmas are ambiguous and even con flicting. According to the Sm-Nd signature, the source of magmas is ob viously crustal, whereas a mantle imprint seems evident from the stabl e-isotope distribution. A model of low degree of crustal partial melti ng under anhydrous conditions of a lower crust enriched in F (and LILE and HFSE) would be likely. Mantle participation would be indirectly i nvoked through F-rich fluids which originated from mantle defluorinati on. Protracted fractionation, volatile-melt unmixing, alkali loss, and contrasted metasomatism along structural traps are discussed as tenta tive explanations for the rare-metal enrichment in the 'biotitite' occ urrences.