Abstract
The Karamea Batholith forms part of the zone of calc-alkaline orogenic batholiths which rim the Pacific continental margin.
In the vicinity of the Victoria Range the batholith is flanked by quartz-rich sandstones, and argillites of the Greenland Group (?-Lower Ordovician). Contact metamorphism is pronounced on the eastern flanks where biotite, andalusite and cordierite have developed; to the west, biotite only is observed, and the present contact is faulted. A major shear zone forms much of the western margin of the Karamea Batholith in the Victoria Range. More recent high angle reverse faulting subparallel to this shear zone has thrust batholithic rocks westwards over Greenland Group and Tertiary sediments.
Banded metapelites and metapsamrnites (Victoria Paragneiss) occur within the batholith in the south of the mapped area, together with minor anatectic leucosome and amphibolite, and rare calc-silicate horizons. Metapelites contain almandine garnet and sillimanite, and rare K-feldspar; Rb-Sr whole rock dating indicates a late Precambrian metamorphism.
Most of the plutonic rocks in the Victoria Range have been grouped into 3 suites. Gneissic granitoids of the Tarn Summit Suite (Devonian) subconcordantly intrude the Victoria Paragneiss, and form the bulk of the batholith. These tonalitic to granitic rocks are characterised by abundant red-brown biotite, which generally defines an ESE trending foliation. The Tobin Metabasite Suite (Triassic) comprises a number of small intrusions with tholeiitic affinities, composed largely of plagioclase-hornblende-biotite. The Rahu Suite (Cretaceous) comprises five relatively massive, steep sided, high-level plutons, several of which have chilled marginal facies. Four of these plutons are further characterised by green biotite, and complexely zoned and twinned plagioclase. The fifth pluton (Desolation Row Granite) is agarnetiferous alkali-feldspar muscovite granite. A prominent circular topographical feature (15 km in diameter) in the Victoria Range is considered to reflect doming and faulting produced during Rahu Suite plutonism.
The Macey Granite is a distinctive fault-bounded unit forming much of the western boundary of the batholith. Euhedral K-feldspar megacrysts and abundant hornblende are characteristic, and render correlation with either Rahu or Tarn Summit Suites difficult.
Composition of plagioclase in the Rahu Suite 'green biotite' granitoids decreases from ca An30 - An15 (while modal biotite decreases from 7.8 - 3.4%), while that in the Desolation Row Granite is albite, An1-3. K-feldspar is present in most granitoids and is maximum microcline generally of late origin. Green hornblende in the Macey Granite is ferro-edenite, while that in the Tobin Epidiorite is magnesia-hornblende. High Fe3+/Fe2+ and Fe3+/Ti produces the green absorption colour characteristic of Rahu Suite biotites. Biotite ∑Fe/∑Fe+Mg ranges from 41 (Tobin Epidiorite) to 89 and is positively correlated with Al. Crystallization of magmatic epidote (Ps25-29), which occurs as euhedral inclusions within Rahu Suite biotite, necessitated high fO2 and Pfluid. In more evolved Rahu Suite magmas high fO2 (and high MnO/FeO) also favoured crystallization of ilmenite, containing up to 40 mol.% MnTiO3, and almandine-spessartine garnet. Lower fO2 and MnO/FeO, and higher Pfluid favoured almandine-rich garnets in more evolved Tarn Summit Suite magmas.
Magnetite (ulvospinel <0.62%) is restricted to the more mafic of the Rahu Suite plutons. Gahnite and columbite occur in the highly evolved Desolation Row pluton, largely as a result of the lack of biotite, in which Zn, Nb and Ta normally reside.
Major and trace element variation diagrams reveal distinct trends for the Rahu and Tarn Summit Suites except at high SiO2 content. Relationships in the system Ab-An-Or are compatible with a single Rahu lineage resulting largely from plagioclase fractionation. However, major element mixing models suggest that, in general, variation in the Rahu suite is the result of progressive partial melting rather than simple (Raleigh) fractionation, although incremental fractionation could produce the variation. Nevertheless in the Desolation Row Granite the only mechanism capable of producing the observed extreme trace element enrichment/depletion is fractional crystallization. Chemical characteristics of the Victoria Range granitoids support the field and petrographic recognition of two distinct suites, and the similarity of this distinction with the I (Rahu Suite) and S (Tarn Summit Suite) type subdivision recognised in S.E. Australia. In particular, rocks of the Rahu Suite are relatively rich in Ca, Sr, Ba, Na and Mn, while Tarn Summit rocks are relatively rich in Fe, Ti, Mg and Rb. Initial 87sr/86 Sr is higher in the Tarn Summit Suite (0.709) than the Rahu Suite (0.706-0.707). Rahu Suite magmas were possibly derived by partial melting of either subduction zone-derived intermediate-basic rocks or Rb-depleted granulite facies rocks near the base of the crust, or immature, possibly volcaniclastic, sedimentary rocks. Tarn Summit granitoids are more likely to have been derived by partial melting of pelitic metasedimentary material within the continental crust.
Hydrothermal alteration is common in granitoid rocks of the Victoria Range; release of Ca from altered plagioclase has enabled complimentary replacement of biotite by a number of Ca-Al silicates, including andradite-grossular garnet and Fe-rich epidote, prehnite and pumpellyite, and Al-rich sphene; in addition to chlorite and K-feldspar.
Sparsely distributed lamprophyre dykes (Cretaceous) intrude both Tarn Summit and Rahu Suites, and trend N-S. Camptonites are restricted to the west of the range and spessartites to the east. Some camptonites have fenitized their granitoid hosts. Composite dykes (emulsion textured basaltic-andesite/ trondhjemite) are restricted to Tarn Summit Suite hosts and trend E-W. The acid phase is considered to represent a melt, derived either by fractional crystallization or melting of basalt, or by melting of granitoid country rocks, subsequently modified by reaction with the basaltic host.