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 anorogenic composite body of two different biotite granite intrusions. They correspond to two batches (with an apparent gap of 10 Ma) of magma with A-type affinities. Sub-, trans- and hypersolvus textures of feldspars are present in the first intrusion, whereas hypersolvus textures dominate in the second. Biotite appears as a late (interstitial or in cluster) 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 inferred differentiation. If a classical fractional crystallization model can be invoked for the first intrusion, a more complicated petrogenetic path has be considered for the second. Biotite-rich sequences or layers up to true 'biotite' bodies (up to 30 vol.% of F-rich, annite-like biotite) occur structurally close to the roof of the second intrusion. Together with their high biotite content, these bodies are characterized by a particularly rich accessory suite, where zircon, a Th-, Ca-rich fluoride, and fluorite are dominant, which is responsible for high 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 in the crystallization sequence (with biotite as an interstitial phase, and miarolitic cavities), in a subvolcanic setting (porphyritic and granophyric textures are present) seems likely. The large increase in most of the HFSE correlates with F. This correlation originated at the magmatic stage. Subsequent fluorine complexing is assumed to have scoured 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 close to the roof of the second intrusion. Zr, Th, and REE enrichment in residual melt appeared as a consequence of the initial alkaline character of the melt and was followed by alkali loss through degassing. Isotopic constraints on the source of magmas are ambiguous and even conflicting. According to the Sm-Nd signature, the source of magmas is obviously crustal, whereas a mantle imprint seems evident from the stable-isotope distribution. A model of low degree of crustal partial melting under anhydrous conditions of a lower crust enriched in F (and LILE and HFSE) would be likely. Mantle participation would be indirectly invoked through F-rich fluids which originated from mantle defluorination. Protracted fractionation, volatile-melt unmixing, alkali loss, and contrasted metasomatism along structural traps are discussed as tentative explanations for the rare-metal enrichment in the 'biotitite' occurrences.