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Evolution of the Sybella Batholith: Petrographic, Geochemical And ResearchOnline@JCU This file is part of the following reference: Hoadley, Elizabeth (2003) Evolution of the Sybella Batholith: Petrographic, geochemical and structural development of an A-type intrusive complex, Northwest Queensland PhD thesis, James Cook University. Access to this file is available from: http://eprints.jcu.edu.au/1346 The author has certified to JCU that they have made a reasonable effort to gain permission and acknowledge the owner of any third party copyright material included in this document. If you believe that this is not the case, please contact [email protected] and quote http://eprints.jcu.edu.au/1346 Chapter 3 Petrography and Mineralogy _____________________________________________________________________________________________________________________________________________ CHAPTER 3 PETROGRAPHY AND MINERALOGY 3.1 INTRODUCTION The evolution of igneous rocks is in part recorded by mineral textures within the intrusives and partly by the mineral chemistry. Study of disequilibrium textures and mineral compositions has been an effective way of determining the role of magma mixing or mingling in silicic to intermediate rocks. The criteria for the identification of magma mingling aside from the occurrence of enclaves has been described by numerous authors (including Vernon 1983; Hibbard 1991; Vernon 1991a; Andersson & Eklund 1994; Wallace & Carmichael 1994; Gencalioglu Kuscu & Floyd 2001) and is summarized here into two categories. (1) Textural or petrographic criteria: including disequilibrium textures such as sieved or dusty plagioclase, rounded and embayed crystals, reaction rims on minerals. (2) Mineral chemistry criteria: including the occurrence of heterogeneous core to rim phenocryst compositions and normal and reverse mineral zoning. The list within the categories is not intentionally extensive or exclusive. The study of the textural development of igneous rocks can identify early stages of crystallization when crystal growth proceeds relatively uninhibited, followed by the linking up of crystals into a framework, and then by the interstitial crystallization of the residual magma. The presence of phenocrysts which enclose small crystals record a snapshot of the early-formed crystals in the magma. The primary mineral assemblage records the effects of pressure, temperature and water content. Compositional and textural features of phenocrysts in the mixed magmas can give clues about the composition and temperatures of the end member magmas (Wallace & Carmichael 1994). 3-1 Chapter 3 Petrography and Mineralogy _____________________________________________________________________________________________________________________________________________ In this chapter the mineralogy, textures and mineral chemistry of the felsic, mafic and intermediate rocks of the Sybella Batholith are presented and evaluated to see if features indicative of mixing are present and consistent with field relationships. 3.2 PETROGRAPHY Modal compositions in terms of quartz, alkali feldspar and plagioclase are plotted in Figure 3.1 and listed in Appendix II. Figure 3.1 shows that the mineralogy of the mafic rocks is dominated by plagioclase, pyroxene, hornblende, biotite, apatite and opaque minerals and the granitic rocks are largely K-feldspar, quartz, plagioclase, biotite ± hornblende and titanite. The rocks identified as hybrids in outcrop have an intermediate composition relative to the mafic and felsic end-members, and commonly contain phenocrysts sourced from the two different end-member magmas. Biotite occurs in all rock types and is magmatic in origin and/or present as a replacement of amphibole. Pyroxene and hornblende occur in the dolerites and some of its hybrids. Apatite and opaque minerals are common accessory minerals in all rock types, while titanite and zircon occur in hybrids and granites. The composition of minerals in these intrusions was determined using an electron probe microanalyser. Each composition represents an average of several grains, each of which is the average of several analyses. Representative microprobe analyses of feldspar, amphibole, biotite and pyroxene of the mafic and felsic rocks are listed in Appendix III, and a summary of the textures and mineral compositions are given in Table 3.1 and Figure 3.2. Mineral classifications are given in Figures 3.3-3.7. 3.2.1 Mafic Intrusive Suite Mafic intrusions that are represented throughout the development of the Batholith are divided into two groups: the Mosses Tank Dolerites and the Mafic Hybrid Complex as discussed in Chapter 2. The intrusive phases of the Mosses Tank Dolerites (in the north of the Batholith) have various textures and mineralogy including biotite and plagioclase porphyritic textures to equigranular fine- to coarse-grained textures. The dolerites are intruded by sheets of microgranite. Interaction between the different phases of dolerite 3-2 Chapter 3 Petrography and Mineralogy Q Figure 3.1 (A) Modal classification of Quartz-rich granitoids geochemically analysed samples 262 60 Symbols Gran te Granite Ton ani odi Kitty plain microgranite Porphyritic granite alite 350 Main phase granite orite Kitty Plain hybrid dolerite 307 Mosses Tank Dolerite Granodiorite intrusive dspar gr Rapakivi granitoid 324 217 505 177 Mafic Hybrid Complex- 291 Country rocks Group 1 520 granodiorite lkali-fel 52 A 314 129 Country rocks Group 2 46 Dolerite enclave 219 183 Mafic Hybrid Complex-diorite 131 312 351 316 514 502 Syenogranite 224 80 226 107 20 229 Quartz-diorite or 51 quartz-gabbro Syenite Quartz Quartz 503 monzonite monzodiorite 315 418 27 322 Diorite or 290 gabbro 18 A 17 10 35 65 90 P KPR-80 KPR-107 SYC-229 ECV MCK-514 ROCKS QRY-418COUNTRY KPD-291 Microgranite KPR-131 KPR-129 Mg granite hybrid KPD-262 KPR-52 Mafic hybrid KPR-46 KPR-51 KPR-27 Mosses Tank KPR-18AND DOLERITE Dolerite KPR-17 KPD-290 MICROGRANITE, HYBRIDS Dolerite enclave QRY-324** MCK-520 Plg QRY-316 Main phase QRY-350** granite Qtz QRY-351** SYC-219** Kfs SYC-183 MCK-505 Bt QRY-307** MCK-502 Hybrid granite/ Hbl SYC-217 Rapakivi SYC-177 Granitoid Pyx QRY-314**AND DIORITE SYC-226 Ttn QRY-312 Intermediate MCK-506 hybrid Ap SYC-224 MAIN PHASE GRANITE, HYBRIDS Mafic Hybrid MCK-503 Opq Complex QRY-315 QRY-322 Dolerite enclave Zrn 0% 20% 40% 60% 80% 100% Figure 3.1 (B) Modal mineralogy of the different rock types of the Sybella Batholith 3-3 Plagioclase K-feldspar Biotite Hornblende Table 3.1 Summary of feldspar, biotite and amphibole compositions in different rock types, Sybella Batholith. (Na,Ca)1Al1-2[Si2-3O8] KAl[Si3O8]K2(Mg,Fe)6Al2[Si6O20](OH)4 (Na,K)0-1Ca2(Mg,Fe,Al)5Si6-7.5Al2-0.5O22](OH)2 An Ab Or An Ab Or Mg No. Mg No. Na+K Core 22 77 1 Pheno core 1 10 89 Core 26-28 Core 14 0.780 350 Main Phase Granite Rim 21-5 79-95 1-0 Pheno rim 1 5 94 Rim 25-29 Rim 15-16 0.824-0.896 MAIN PHASE SUITE PHASE MAIN Mantle 24-32 75-67 1 Matrix grain rim 1 7 92 Rim 22 77 1 Pheno Core 1 5 94 Rim 25 Core 18 0.762 242 Mantle 3 80 17 Pheno Rim 0 7 92 Rim 17 0.695 Kfspheno kfs overgrowth1693 Matrix mineral 1 7 93 Granitoids Rim and Matrix 28 71 1 Core 1 11 88 Core 42 Core 38-40 0.694-0.701 Rapakivi Rapakivi 314 0.601-0.750 25 74 1 Rim 0 9 91 Rim 42-44 Rim 38-34 Core and phenocrysts 33 66 1 inclusion 40 35 65 0 Mafic hybrid hybrid Mafic MAFIC HYBRID MAFIC Core 32 68 1 Core 1 16 83 Core 37 Rim 36-38 0.659-0.723 COMPLEX 410 Rim 29 71 1 Rim 0 12-14 88-86 Rim 39 Core 37 62 1 3-4 Rim 35 65 0 Diorite 315 Core 44-49 55-50 1 Matrix mineral 1 9 90 Core 39 Core 46 0.287 Rim 35-52 64-47 1 Rim 38-41 Rim 43 0.586 Microgranite hybrid hybrid MICROGRANITE AND MICROGRANITE Rim 22 63 15 5 8 86 Core 47 Rim 37-42 0.44-0.71 129 12 87 0 2 5 93 Rim 47-49 HYBRIDS 32 68 0 1 13 87 Mg granite hybrid Mg granite Pheno core 73-74 27-26 2-1 Kfs pheno core 0 11 88 Core 38-39 Cluster core 39 0.504-0.615 Pheno rim 39-41 60-58 1 Kfs pheno rim 1 16 84 Rim 38-41 Rim/core 37-42 0.545-0.692 281 281 18 Mantle 39 61 1 Kfs pheno transect 12 28 60 Matrix mineral 36-41 64-58 1-0 Core 1 12 87 Rim 0 10 90 hybrid Mafic DOLERITE MOSSES Core 44 55 1 Core 1 14 85 Matrix mineral 43 Inclusion 42 0.590 TANK Rim 43 56 1 Rim 43 0.478 Dolerite Core 63-57 37-42 0 Core 44-45 Core 62 0.201-0.326 Rim 53-48 46-51 0 Alt rim 56 0.147 Chapter 3 Petrography and Mineralogy FeO (a) Main Phase granite Hbl Opx MAIN PHASE SUITE Main phase granite: 350, 242 Bt Cpx Rapakivi Granitoid: 314 Mafic Hybrid Complex: 410 Diorite: 315 MICROGRANITE SUITE Microgranite hybrid: 281 Microgranite hybrid: 129 Mafic hybrid: 281 Mosses Tank Dolerite: 18 Feldspars Na22 O + K O MgO Al23 O Al23 O MAIN PHASE MICROGRANITE (b) SUITE SUITE (c) Pl Pl Bt Bt Kfs Kfs Hbl Hbl Pyx Pyx Na22 O + K O CaO Na22 O + K O CaO MAIN PHASE SUITE MICROGRANITE SUITE 18 18 (d) (e) Kfs 15 15 Kfs 12 12 Bt Pl Bt Pl 9 9 22 22 Na O + K O 6 Na O + K O 6 Hbl Hbl 3 3 Pyx Pyx 0 0 30 35 40 45 50 55 60 65 70 30 35 40 45 50 55 60 65 70 SiO2 (wt %) SiO2 (wt %) Figure 3.2 Mineral composition data illustrated on ternary and binary diagrams (a) Na2 O - FeO - MgO plot highlights the Fe-rich composition of the biotite and amphibole minerals in the Main Phase granite; (b) Na22 O+K O - Al 23 O - CaO plot of the Main Phase Granite Suite illustrates the intermediate nature of the interpreted hybrids.
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