Petrogenesis and Tectono-magmatic Evolution of S-type and A-type granites in the New England Batholith

A Thesis submitted to The University of Newcastle for the Degree of Doctor of Philosophy

B. Landenberger B.Sc. (Hons) July 1996

Frontispiece: View looking east from the Devil’s Forehead (near the summit of Chaelundi Mountain in the Guy Fawkes National Park) over the headwaters of in Chaelundi State Forest. The foreground outcrop forms part of the Chaelundi Complex A- type granite suite. I hereby certify that the work embodied in this thesis is the result of original research and has not been submitted for a higher degree to any other University or Institution.

(Signed)______ACKNOWLEDGMENTS

Several people have lent helping hands during the course of this project with technical and logistical problems, while others have helped to prevent the onset of insanity. Bill Collins and Robin Offler supervised the project and have been invaluable help in clarifying ideas and helping with organisational aspects of the thesis work. Bill Collins provided fervent inspiration in times when things seemed to be going in circles. Discussions with Paul Dirks, Sue Keay, Terry Farrell, Martin Hand, Dick Flood, Stirling Shaw, and Ron Vernon have also aided in developing the ideas presented herein.

Many staff members of the Geology Department have provided technical assistance and advice, including Richard Bale, Esad Krupic, Hope Ruming, Geraldene MacKenzie and Jan Crawford. I also thank Doug Todd for his help with XRF analysis and Dave Phelan for his help with microprobe analysis. Isotopic analyses were carried out at the Centre for Isotope Studies (CIS - North Ryde) under the guidance of Dave Whitford, Bo Zhou and Steve Craven. Stereographic projections were produced with the computer program GeOrient which was kindly supplied by Rod Holcombe (Department of Earth Sciences, University of Queensland).

During the first three years of the project, I was supported by an Australian Postgraduate Research Award. Funding for external analytical work was supported by several ARC and University (SRC) research grants to Bill Collins and Robin Offler.

Finally, I would like to thank my wife Debbie for her help, support, and most of all patience and understanding over the last six years.

For Debbie, Justine and Daniel -i-

TABLE OF CONTENTS

LIST OF FIGURES...... v

LIST OF PLATES...... ix

LIST OF TABLES...... x

ABSTRACT...... xi

CHAPTER 1. INTRODUCTION ...... 1 1A1 Preamble - Physiography of the New England Tableland...... 1 1A2 Regional geological setting ...... 2 1A3 A Brief Tectonic History ...... 4 1A3A1 Pre- late Carboniferous ...... 4 1A3A2 Post- late Carboniferous ...... 5 1A4 Previous studies of the New England Batholith ...... 6 1A5 Aims of the project...... 8

CHAPTER 2. STRUCTURAL RELATIONS OF THE HILLGROVE PLUTONIC SUITE 10 2A1 Introduction ...... 10 2A2 Structural Setting ...... 12 2A2A1 Distribution of the Hillgrove Suite ...... 12 2A2A2 Structural history of the Tia Complex...... 13 2A3 Structural Framework...... 14 2A3A1 A definition of domains...... 15 2A3A2 Structural continuity outside the Tia Complex - Macroscopic structural features...... 16 2A3A3 Meso- and Microscopic Deformation Features...... 19 2A3A4 A correlation of post-accretion deformation structures - the regional perspective...... 32 2A4 Conclusions and Regional Implications...... 43 -ii-

CHAPTER 3. AGE RELATIONS OF THE HILLGROVE PLUTONIC SUITE ...... 47 3A1 Introduction ...... 47 3A2 Previous geochronology of the ...... 49 3A3 Structural sequence associated with the Hillgrove Suite - a brief review ..... 53 3A4 Sample Selection, Analytical Methods ...... 54 3A4A1 Zircon U-Pb...... 54 3A4A2 Biotite Rb-Sr ...... 57 3A6 Discussion...... 61

CHAPTER 4. PETROGENESIS OF THE HILLGROVE PLUTONIC SUITE...... 66 4A1 Introduction ...... 66 4A1A1 S-type granites in the SNEFB ...... 67 4A1A2 Previous petrogenetic studies of the Hillgrove Suite ...... 67 4A2 Defining the Hillgrove Suite ...... 68 4A2A1 Previous definitions of the Hillgrove Suite ...... 68 4A2A2 Definition of the Hillgrove Supersuite ...... 71 4A3 Petrography & Mineral Chemistry ...... 78 4A3A1 General Petrography of Hillgrove Supersuite granitoids ...... 78 4A3A2 Biotite ...... 82 4A3A3 Other ferromagnesian phases: Amphiboles & pyroxenes...... 85 4A3A4 Peraluminous phases other than biotite: garnet, cordierite, muscovite...... 87 4A3A5 Oxides and other opaque phases ...... 90 4A3A6 Plagioclase...... 91 4 A3A7 Alkali Feldspar...... 92 4A3A8 Quartz...... 93 4A4 Geochemistry ...... 96 4A4A1 Defining the geochemical characteristics - variations within the Hillgrove Supersuite, and comparisons with other granitoids...... 96 4A4A2 The Bakers Creek Suite...... 110 4A4A3 Rocks of accretion complex ...... 114 4A5 Petrogenetic Constraints...... 115 4A5A1 Parental magmas of the Hillgrove Suite ...... 115 -iii-

4A5A2 Identifying the source rocks...... 118 4A5A3 Quantification the partial melting process...... 133 4A5A4 Contamination within the Hillgrove Supersuite - the possibilities...... 140 4A6 Conclusions...... 143

CHAPTER 5. PETROGENESIS OF THE CHAELUNDI COMPLEX A-TYPE GRANITOID SUITE: DERIVATION BY PARTIAL MELTING OF A DEHYDRATED CHARNOCKITIC LOWER CRUST...... 144 5A1 Introduction ...... 144 5A2 Geological Setting ...... 145 5A3 Petrography of the Suites ...... 149 5A3A1 The I-type suite ...... 149 5A3A2 The A-type suite...... 155 5A4 Geochemistry ...... 156 5A5 Petrogenetic Constraints...... 161 5A5A1 Fractionation ...... 161 5A5A2 Parental magmas ...... 172 5A5A3 Geochemical contrasts between the Chaelundi Complex I- and A-type parental magmas...... 172 5A5A4 Nature of the source rocks...... 174 5A6 Petrogenetic Models...... 175 5A6A1 Previous Models ...... 175 5A6A2 Charnockitization of the lower crust and generation of A-type magmas176 5A7 Conclusions...... 184

CHAPTER 6. PETROGENESIS OF BASALTIC ENCLAVES IN THE A-TYPE WOODLANDS QUARTZ MONZONITE ...... 185 6A1 Introduction ...... 185 6A2 Geological Setting ...... 186 6A3 Microgranitoid enclaves in A-type granites of the New England Batholith ...... 188 6A4 Field relationships of the Woodlands Quartz Monzonite and its enclaves .... 188 -iv-

6A4A1 Age and intrusive relationships...... 188 6A4A2 Outcrop detail ...... 190 6@5 Petrography and Mineral Chemistry ...... 194 6A5A1 The host quartz monzonite...... 194 6A5A2 Enclave matrix...... 194 6A5A3 Enclave phenocrysts ...... 197 6A5A4 Enclave xenocrysts ...... 202 6A5A5 Pyroxene thermometry...... 203 6A6 Geochemistry ...... 204 6A6A1 The host quartz monzonite...... 204 6A6A2 Enclaves...... 209 6@7 Enclave Petrogenesis ...... 210 6A7A1 Restite, Cumulate or Mingled Magma? ...... 210 6A7A2 Composition of the enclave parent magma...... 215 6A7A3 Post-emplacement chemical evolution of enclave magmas ...... 218 6A8 Conclusions...... 231 6A8A1 A model of enclave formation for the Woodlands example ...... 231 6A8A2 General implications for the origin of microgranitoid enclaves .... 236

CHAPTER 7. A TECTONIC SYNTHESIS...... 238

REFERENCES ...... 242

APPENDICES...... 266 Appendix A: Analytical methods...... 267 Appendix B: Structural data...... 270 Appendix C: Modal data for granitoids...... 273 Appendix D: Mineral analyses (Electron-microprobe)...... 277 Appendix E: Whole-rock geochemical analyses...... 290 Appendix F: Isotopic data ...... 304 Appendix G: Catalogued sample list with grid references...... 306 -v-

LIST OF FIGURES

Figure 1A1. Simplified map of the southern New England Fold Belt...... 3 Figure 2A1. Areal distribution of the Hillgrove Suite and defined structural domains...... 11

Figure 2A2. D3 form-surface map for accretion complex metasediments...... 34

Figure 2A3. D5 form-surface map for accretion complex metasediments...... 36 5 Figure 2A4. Contoured stereoplot of poles to S5 overlain with L5 stretching lineations...... 37

7 Figure 2A5. Stereoplot of poles to D7 mylonite shear (C) planes overlain with L7 stretching lineations...... 37

Figure 2A6. Map of D7 shear zones in Hillgrove Suite granitoids and metasediments. . . 40 Figure 2A7. Unwinding the New England orocline...... 45 Figure 3A1. Geological map of southern New England Fold Belt...... 48 Figure 3A2. (a) Standard isochron plot of whole-rock Rb-Sr data for Hillgrove Suite granitoids. (b) Best isochron plot of whole-rock Rb-Sr data for Hillgrove Suite granitoids...... 51 Figure 3A3. Concordia plot of zircon U-Pb data from the Tia Granodiorite and Abroi Granodiorite...... 55 Figure 3A4. Map of the Hillgrove Plutonic Suite and associated Bakers Creek Suite with sample locations and Rb-Sr biotite ages ...... 59 Figure 3A5. Contour map of the biotite Rb/Sr ages from Hillgrove Suite granitoids not

mylonitized during D7...... 63 Figure 3A6. Schematic cross-sections of the Wollomombi Zone...... 65 Figure 4A1. Map of plutons of the Hillgrove Supersuite...... 73 Figure 4A2. (a) Streckheisen plot of Hillgrove Supersuite and Bundarra Suite granitoids. (b) Modal variation within the Hillgrove suite...... 79 Figure 4A3. Biotite compositions of Hillgrove Supersuite and Bundarra Suite granitoids...... 84 Figure 4A4. (a) Classification of Rockisle Suite amphiboles. (b) Rockisle Suite amphibole compositions and Bakers Creek pyroxene compositions plotted in the pyroxene quadrilateral...... 86 -vi-

Figure 4A5. Garnet, cordierite, and ilmenite compositions from the Hillgrove Suite and Bundarra Suite plotted in terms of ternary Fe-Mg-Mn contents...... 89 Figure 4A6. Ternary plot of feldspar compositions for Hillgrove Supersuite and Bundarra Suite granitoids...... 89 Figure 4A7. Major element Harker plots...... 98 Figure 4A8. Trace element Harker plots...... 101 Figure 4A9. Molecular Ca-Na-K and ACF plots...... 104 Figure 4A10 Inter-element ratio Harker plots...... 105 Figure 4A11. REE spidergram...... 109 Figure 4A12. AFM and tectonic discrimination plots for gabbros of the Bakers Creek Suite.112 Figure 4A13. Q-Ab-Or and An-Ab-Or plots...... 124 Figure 4A14. Batch partial melting trends...... 127

87 86 Figure 4A15. eNd vs Sr/ Sri plot for granitoids and gabbros of the Hillgrove Supersuite and accretion complex metasediments...... 131 Figure 4A16. Harker plots of residuum compositions calculated by major element modelling.138 Figure 5A1. (a) Geological map of the southern New England Fold Belt showing the outcrop area of the granitoids younger than 240 ma...... 146 Figure 5A1. (b) Geological map of the Chaelundi Complex ...... 148 Figure 5A2. Streckheisen plot of representative modes from the two suites of the Chaelundi Complex...... 150 Figure 5A3. Plot of biotite compositions for the A-type suite...... 152 Figure 5A4. Plot of hornblende compositions for the A-type suite...... 153 Figure 5A5. Ternary feldspar plot for the A-type suite...... 154 Figure 5A6. Selected major elements plots...... 157 Figure 5A7. Selected trace elements plots...... 158 Figure 5A8. REE - chondrite normalized plots ...... 160 Figure 5A9. Plot illustrating the combined effect of plagioclase and orthoclase fractionation on Ba and Eu contents within the A-type suite...... 168 Figure 5A10. Selected plots of inter-element ratios vs silica...... 171 Figure 5A11. Y vs Nb tectonic discrimination diagram for both suites ...... 173 -vii-

Figure 5A12. Ce/Nb vs Y/Nb discrimination plot for the two major subgroups of A-type granites ...... 173 Figure 5A13. Schematic representation of partial melting processes during I-type magma ...... 180 Figure 5A14. Spidergram plots comparing the A-types and average I-type of the Chaelundi Complex with C-type magmas...... 183 Figure 6A1. Geological map showing the distribution of leucoadamellites and related rocks in the southern New England Fold Belt...... 187 Figure 6A2. Geological map of the Kookabookra - Wards Mistake area ...... 189 Figure 6A3. Mineral chemistry plots for enclaves of the Woodlands Quartz Monzonite and host monzonite...... 198 Figure 6A4. Pyroxene quadrilateral plot of enclave clinopyroxene and orthopyroxene phenocrysts, and their secondary alteration products...... 199 Figure 6A5. Classification of both primary matrix amphibole and secondary amphiboles after clinopyroxene phenocrysts ...... 199 Figure 6A6. Ternary plot of feldspar compositions from the Woodlands Quartz Monzonite and enclaves...... 200 Figure 6A7. Pyroxene thermometry of enclave phenocrysts from the Woodlands Quartz Monzonite...... 200 Figure 6A8. Major element plots for Woodlands Quartz Monzonite and enclaves...... 205 Figure 6A9. Trace element plots for Woodlands Quartz Monzonite and enclaves...... 206 Figure 6A10. Inter-element ratio plots for Woodlands Quartz Monzonite and enclaves...... 207 Figure 6A11. REE chondrite normalized plot for Woodlands Quartz Monzonite and enclaves.208 Figure 6A12. gNd vs initial 87Sr/86Sr plot of isotopic analyses of the Woodlands Quartz Monzonite, enclaves, and a single enclave clinopyroxene separate...... 214 Figure 6A13. Tectonic discrimination diagrams for Woodlands enclaves...... 217 Figure 6A14. Mg# vs XCa and ASI vs Mg# plots showing the relationship between bulk enclave compositions, enclave matrix and phenocrysts...... 228 Figure 6A15. Schematic representation of the three stage model for formation of the Woodlands enclaves...... 233 -viii-

Figure 7A1(a-c). Schematic profiles of the southern New England Fold Belt from early Carboniferous to late Carboniferous...... 239 Figure 7A1(d-e). Schematic profiles of the southern New England Fold Belt from early Permian to middle Triassic...... 241 -ix-

LIST OF PLATES

Plate 2A1. D5 macro- and mesoscopic features of metasediments from the Wollomombi Zone.20

Plate 2A2. D5 microstructures in metasediments...... 22

Plate 2A3. Mesoscopic features of D5 D7 and D8 structures in Hillgrove Suite granitoids. 25

Plate 2A4(a-d). Microscopic features of D7 in Hillgrove Suite granitoids...... 29

Plate 2A4(e-h). Microscopic features of D7 in Hillgrove Suite granitoids...... 30 Plate 4A1. Photomicrographs of Hillgrove Supersuite granitoids and associated rocks...... 81 Plate 5A1. Photomicrographs comparing the microstructure of the mafic end-members of the two suites ...... 151 Plate 6A1. Outcrop features of enclaves in the Woodlands Quartz Monzonite...... 191 Plate 6A2. Outcrop detail of mafic microgranitoid enclaves...... 193 Plate 6A3(a-d). Woodlands Quartz Monzonite - Enclave microstructure...... 195 Plate 6A3(e-h). Woodlands Quartz Monzonite - Enclave microstructure...... 196 -x-

LIST OF TABLES

Table 1A1 Simplified summary of the New England Batholith...... 7 Table 2A1. Distinguishing characteristics of accretion complex and Permian basin metasediments...... 27 Table 2A2. Comparative timing of structural and magmatic events throughout the Tablelands Complex...... 33 Table 3A1 Summary of previous geochronological work on the Hillgrove Suite and associated rocks...... 50 Table 3A2. Biotite Rb-Sr data from Hillgrove Suite granitoids and high grade metasediments. 60 Table 4A1. Characteristics of S- and I-type granites...... 66 Table 4A2. Characteristics of Hillgrove Supersuite and Bakers Creek Suite members...... 72 Table 4A3 Distinctive mineralogy of the Hillgrove, Rockisle and Bundarra plutonic suites ...... 80 Table 4A4. Comparative major and trace element chemistry for LFB I-type, LFB S-type and New England Batholith I-type granites, with the Hillgrove, Rockisle and Bundarra Suite granites...... 107 Table 4A5. Comparison of the average analysis of Bakers Creek Suite gabbros, with mid-ocean -ridge, island arc calc-alkaline and island arc tholeiitic basalts...... 111 Table 4A6. Major element modelling...... 137 Table 4A7. Trace Element Modelling ...... 140 Table 5A1. Comparative mineralogy and geochemistry of the I- and A-type suites...... 156 Table 5A2 Major element modellingA ...... 163 Table 5A3. Trace element Modelling ...... 165 Table 5A4. Rare-earth-element Modelling ...... 167 Table 6A1. Summary of isotopic results from the Woodlands Quartz monzonite and enclaves 213 Table 6A2. Modelling results - Woodlands enclaves ...... 222 Table 6A3. Comparison of average bulk (whole-rock, XRF) enclave compositions and matching average matrix compositions...... 226 -xi-

ABSTRACT

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 0A7048 to 0A7070, gNd +2 to

-1, high Na2O and low ASI), explains the unusual character of Hillgrove Supersuite granitoids, which are isotopically primitive (87Sr/86Sr 0A7040 to 0A7065, gNd -1 to +4), only mildly peraluminous (ASI 1A00 - 1A15), and relatively high in Na2O (3 - 4%) compared to -xii- 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 0A7040, gNd +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 0A7048 to 0A7070, gNd +2 to -1) and coeval gabbros of the Bakers Creek Suite (87Sr/86Sr 0A7027, gNd +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 gNd 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 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 -xiii- 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 a conditions during partial H2O -xiv- 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 900EC, 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.