- Title
- Petrogenesis and tectono-magmatic evolution of S-type and A-type granites in the New England Batholith
- Creator
- Landenberger, B.
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 1996
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The late Carboniferous heralded a fundamental change in tectonic and magmatic styles within the southern New England Fold Belt (SNEFB). Westward migration of arc magmatism during the early and middle Carboniferous, was ensued by rapid easterly migration during the late Carboniferous, establishing a new arc within the original accretion complex (Tablelands Complex) of the SNEFB. This event was accompanied by the onset of high-T/low-P metamorphism and uplift in parts of the accretion complex. Subduction-accretion fabrics developed earlier in the Carboniferous (D1-D2) were initially overprinted by D₃ (~311 Ma) during major uplift of the southern Tia Complex. Biotite grade D₃ fabrics were in turn folded during D₄. This thermal perturbation culminated with intrusion of granitoids of the Hillgrove Supersuite and gabbros and diorites of the Bakers Creek Suite (of island arc tholeiite affinity) in the latest Carboniferous. Intrusion of these arc magmas was accompanied by further compressional deformation (D₅) causing uplift of the entire Wollomombi Zone along its eastern margin. Preliminary zircon U-Pb ages presented herein, provide the first tight constraints on the emplacement and crystallization of the Hillgrove Supersuite granitoids, and also the D₅ deformation, establishing a latest Carboniferous age. Geochemical characteristics, combined with Sr and Nd isotopic compositions of granitoids and various potential source rocks, provide tight constraints on possible sources for the Hillgrove Supersuite granitoids. Only volcanogenic greywackes of intermediate composition (~65% SiO₂) overlap with the isotopic composition of the granitoids, inferring that these metasediments are the most likely source. Calculated melt fertilities of various potential source rocks (based on the proportional component of the ternary Q-Ab-Or minimum melt composition at 5 kb) also indicate that these intermediate greywackes are the most likely sources to produce large volumes of partial melt. Significantly, the isotopic characteristics and calculated melt fertilities preclude the involvement of pelites and felsic greywackes (~70%SiO₂), which have previously been inferred as granitoid sources. The isotopic and chemical immaturity of these sediments (⁸⁷Sr/⁸⁶Sr 0.7048 to 0.7070, εNd+2 to -1, high Na₂O and low ASI), explains the unusual character of Hillgrove Supersuite granitoids, which are isotopically primitive (⁸⁷Sr/⁸⁶Sr 0.7040 to 0.7065, εNd -1 to +4), only mildly peraluminous (ASI 1A00 - 1A15), and relatively high in Na₂O (3 - 4%) compared to most S-types. Major and trace element modelling indicate that the more mafic magmas (68-70% SiO₂) of the suite were produced by ~48% partial melting of the intermediate greywacke source, under water-undersaturated conditions involving biotite breakdown at granulite facies conditions and mid crustal depths (~5 kb). Isotopic and chemical variability within the Hillgrove Supersuite demands that two additional sources have contributed to magma formation. The more isotopically and chemically primitive granites of the Rockisle Suite (⁸⁷Sr/⁸⁶Sr 0.7040, εNd +4.0), which form ~5% of the Hillgrove Supersuite, have a bulk chemistry which deviates from the main Hillgrove Suite, with higher CaO, Al₂O₃, TiO₂ and lower K₂O, ΣFeO contents. These granites plot on an isotopic mixing curve between intermediate greywacke (⁸⁷Sr/⁸⁶Sr 0.7048 to 0.7070, εNd +2 to -1) and coeval gabbros of the Bakers Creek Suite (⁸⁷Sr/⁸⁶Sr 0.7027, εNd +9.5). Accordingly, mantle-derived magmas are considered to have been a contributor to the most primitive granites. Another possible minor magma source are seawater-altered metabasalts, which are common in the deeper parts of the accretion complex. These metabasalts are likely to have undergone small degrees of partial melting (via amphibole breakdown), contributing a minor melt component to the primary S-types, thus causing an isotopic shift towards higher εNd and higher initial ⁸⁷Sr/⁸⁶Sr. After the D₅ event and intrusion of the Hillgrove Supersuite, compressional tectonics gave way to rifting in the early Permian, with the development of early Permian basins such as the Nambucca and Manning basins within the Tablelands Complex, and the Sydney Basin further to the west. During the early and middle Permian, there was little manifestation of arc magmatism within the SNEFB. However, intrusion of the Bundarra Plutonic Suite, and extrusion of bimodal volcanics in the rift basin sequences, probably represent inter-arc or back-arc rifting during establishment of an intra-oceanic island arc further to the east. Compressional tectonics dominated the late Permian, with climactic deformation during the Hunter-Bowen Orogeny. This event initially generated large-scale folding of earlier fabrics within the accretion complex (F₆). On a regional scale, this folding was related to development of the Texas - Coffs Harbour Orocline and dispersal of discrete structural blocks within the Tablelands Complex. F₆ folds in the Wollomombi Zone and S₁ cleavage in the adjacent early Permian Nambucca Block, are both truncated by D₇ ductile shear zones which represent the culmination of this deformation. D₇ involved westward tilting of the entire Wollomombi Zone with up to 8 km of uplift, along mylonite zones which truncated many plutons of the Hillgrove Supersuite. Rb-Sr dating of biotite from high-grade D₇ mylonite zones constrain the age of D₇ uplift to the late Permian (258-266 Ma). Crustal tilting during D₇ has also produced the large range of Rb-Sr biotite ages for Hillgrove Supersuite granitoids not directly affected by D₇ mylonitization. Slow thermal relaxation after the high-T/low-P event which accompanied intrusion of the Hillgrove Supersuite, combined with crustal tilting at ~260 Ma, produced a pattern of biotite ages which decrease from west to east, as the major late Permian shear zones are approached. The oldest biotite ages in this range (296 Ma) are within error of the age of granitoid intrusion, while the youngest ages (257 Ma) record the age of uplift. This climactic deformation was followed by re-establishment of arc-related volcanism in the early Triassic, which involved minor crustal extension and major plutonism, with intrusion of I- and A-type granites of the New England Batholith. I- and A-type granites of the Chaelundi Complex were generated at this time, in a subduction-related tectonic setting. Although isotopic ages of the suites are indistinguishable (233-235 Ma), field relations indicate that the A-type is younger. The most mafic granitoids from each suite have similar silica contents (66-68% SiO₂), slightly LREE enriched patterns without Eu anomalies, low Rb/Sr and K/Ba ratios, and high K/Rb ratios, suggesting that both represent parental magmas. The A-type is distinguished mineralogically by abundant orthoclase and sodic plagioclase (total >60%), ferro-hornblende, annite and allanite. In contrast, the I-type has more hornblende and biotite, which are more magnesian in composition, and less feldspar. The parental magmas of both suites have many similar geochemical characteristics, although the A-type has slightly higher alkalis, Zr, Hf, Zn and LREE, and lower CaO, MgO, Sr, V, Cr, Ni and Fe³⁺/ΣFe. The geochemical features characteristic of leucocratic A-type granites, such as high Ga/Al, Nb, Y, HREE and F contents, are only manifest in the more felsic members of the A-type suite. These features were produced by ~70% fractional crystallization of feldspar, hornblende, quartz and biotite. Both granite suites were generated by water-undersaturated partial melting of a similar source, but the A-type parent magma resulted from lower ⍺H₂O conditions during partial melting. Generation and rapid ascent of the earlier I-type magma during disequilibrium partial melting produced a relatively anhydrous, but not refractory, charnockitic lower crust. Continued thermal input from mantle-derived magmas, during ongoing subduction, partially melted the ‘charnockitized’ lower crust at temperatures in excess of 900°C, to produce A-type magmas. As the I- and A-type granites intruded penecontemporaneously, a tonalitic source model for genesis of the Chaelundi A-type, is untenable. Basaltic enclaves preserved in the nearby Woodlands Quartz Monzonite, also of A-type affinity, provide evidence that basaltic magmas of island arc affinity were still providing the heat source necessary for partial melting in the lower crust during the Triassic. Combined petrographic, geochemical and isotopic data provide unequivocal evidence that the enclaves present in the Woodlands Quartz Monzonite, originated as a coeval basaltic magma, that mingled with the host quartz monzonite. The preserved basaltic phenocryst assemblage(augite + hypersthene + calcic plagioclase) and the geochemical character of the enclaves, suggest that the parent magma was of high-alumina basalt affinity. Variations in chemistry and mineralogy of the enclave suite are the result of several magmatic differentiation processes which have affected the original basaltic magma. Modelling suggests that internal fractional crystallization was the primary process responsible for differentiation of the enclave suite, together with concomitant processes of physical exchange with the host granitoid. These processes include diffusional exchange at the molecular scale, as well as exchange of phenocrysts, and minor late metasomatism. The unique preservation of the basaltic phenocryst assemblage and the basaltic geochemical character of these enclaves are the result of arrested hybridism. Although the enclaves preserved in the Woodlands Quartz Monzonite are an unusual example, their similarities to enclaves present in other granitoid types (particularly I-types) in the New England Batholith and elsewhere, suggest that this model may be applied to many examples of microgranitoid enclaves.
- Subject
- Carboniferous; Hillgrove Supersuite; geochemical; volcanism; Tablelands complex
- Identifier
- http://hdl.handle.net/1959.13/1404238
- Identifier
- uon:35297
- Rights
- Copyright 1996 B. Landenberger
- Language
- eng
- Full Text
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