S-type Magmas Research Articles

Mixing of cordierite granitoid and pyroxene gabbro, and fractionation, in the Santa Olalla tonalite (Andalucia)

The asymmetricaly zoned Santa Olalla pluton consists of several facies; gabbro; a tonalitic group that includes both hornblende diorite and cordierite-bearing quartz monzonite; granitic dykes; and enclaves. Enclaves are an ubiquitous component and in places they occur in swarms. The tonalites and enclaves display extremely abundant disequilibrium microstructures: pyroxene replaced by actinolite; actinolite by hornblende; hornblende by biotite; biotite by hornblende; and both calcic plagioclase and biotite phenocrysts have been corroded in a disequilibrium melt phase. Most of these microstructures can develop during the evolution of a single magma, but the replacement of biotite suggests that crystal-melt disequilibrium through magma mixing is responsble. Hornblende and actinolite occur in clots enclosing pyroxene. The growth of actinolite was probably metastable at magmatic temperatures, and replacement of pyroxene was controlled by diffusion of Al. Subsequently, hornblende replaced actinolite, with a melt phase present. Plagioclase microstructures and compositions are consistent with batches of phenocrysts immersed in a disequilibrium melt that resulted from a bulk compositional change in the magma, with which they re-equilibrated. Bulk-rock geochemistry indicates that the series gabbro through tonalite to granitic dykes cannot be simply related by a single fractionation trend. Gabbros vary by pyroxene fractionation. The tonalites are a mixture of a cordierite-bearing S-type magma and a somewhat fractionated pyroxene gabbro: thus, it is H(Hybrid)-type plutonism, not I-type. The granitic dykes developed after mixing, by segregation of a residual melt from the hybrid tonalites. There is evidence of some degree of fractionation of the tonalite facies after mixing: the presence of aplites, granitic dykes, and divergence of incompatible trace elements from simple mixing ratios. No variation can be attributed to restite unmixing.

Oligocene and miocene continental sedimentation, tectonics, and S-type magmatism in the southeastern Andes of Peru (Crucero Basin): geodynamic implications

The Crucero Basin, located north of Lake Titicaca (70°W, 14°20's), was probably connected with the Altiplano endoreic basin since Oligocene time. During the Oligocenne and Miocene, this basin was filled by nearly 1000 meters of fanglomerate and lacustrine sediments —e.g., the Cayconi Formation, which unconformably overlies upper Paleozoic and Cretaceous rocks and is unconformably overlain by fanglomerates and palustrine sediments of Pliocene age. Basic and silicic volcanic rocks are interbedded and associated with the Cayconi Formation. Major element analyses indicate intracrustal melting for the silicic volcanic rocks, whereas the basic volcanic seem to be mantle derived. K-Ar dating of these volcanic rocks yielded ages ranging from 25.9 Ma to 15.5 Ma. Thus, in the Cordillera Oriental, magmatism is at least as old as late Oligocene.

Some supracrustal (S-type) granites of the Lachlan Fold Belt

ABSTRACTS-type granites have properties that are a result of their derivation from sedimentary source rocks. Slightly more than half of the granites exposed in the Lachlan Fold Belt of southeastern Australia are of this type. These S-type rocks occur in all environments ranging from an association with migmatites and high grade regional metamorphic rocks, through an occurrence as large batholiths, to those occurring as related volcanic rocks. The association with high grade metamorphic rocks is uncommon. Most of the S-type granites were derived from deeper parts of the crust and emplaced at higher levels; hence their study provides insights into the nature of that deeper crust. Only source rocks that contain enough of the granite-forming elements (Si, Al, Na and K) to provide substantial quantities of melt can produce magmas and there is therefore a fertile window in the composition of these sedimentary rocks corresponding to feldspathic greywacke, from which granite magmas may be formed.In this paper, three contrasting S-type granite suites of the Lachlan Fold Belt are discussed. Firstly, the Cooma Granodiorite occurs within a regional metamorphic complex and is associated with migmatites. It has isotopic and chemical features matching those of the widespread Ordovician sediments that occur in the fold belt. Secondly, the S-type granites of the Bullenbalong Suite are found as voluminous contact-aureole and subvolcanic granites, with volcanic equivalents. These granites are all cordierite-bearing and have low Na2O, CaO and Sr, high Ni, strongly negative εNd and high 87Sr/86Sr, all indicative of S-type character. However, the values of these parameters are not as extreme as for the Cooma Granodiorite. Evidence is discussed to show that these granites were derived from a less mature, unexposed, deeper and older sedimentary source. Other hypotheses such as basalt mixing are discussed and can be ruled out. The Strathbogie Suite granites are more felsic but all are cordierite-bearing and have chemical and other features indicative of an immature sedimentary source. They are closely associated with cordierite-bearing volcanic rocks. The more felsic nature of the suite results in part from crystal fractionation. It is suggested that the magma may have entered this “crystal fractionation” stage of evolution because it was a slightly higher temperature magma produced from an even less mature sediment than the Bullenbalong Suite. The production of these S-type magmas is discussed in terms of vapour-absent melting of metagreywackes involving both muscovite and biotite. The production of a magma in this way is consistent with the low H2O contents and geological setting of S-type granites and volcanic rocks in the Lachlan Fold Belt.

Sulfide and oxide minerals from S-type and I-type granitic rocks

In the Ohmine I-type and S-type granitic rocks of Miocene age, Kii peninsula, central Japan, ilmenite, rutile, pyrrhotite, pyrite and chalcopyrite are common, but no magnetite is present. The primary paragenetic relations of these minerals are determined by their modes of occurrence as inclusions within major plagioclase, orthoclase, quartz, hornblende and biotite, although their parageneses changed during cooling. Significant subsolidus reactions for sulfide minerals took place in both the groundmass and the inclusions until the rocks cooled below 300°C. The loss of S and Cu through silicate crystals was too easy to preserve the chemistry of sulfides included in silicates. However, the removal of Ni through silicate crystals as well as along grain boundaries was restricted, as indicated by the fact that the distribution frequency of Ni-bearing pyrrhotite is higher in inclusions than in the groundmass, and significantly higher in I-type cognate enclaves than in host I-type rocks. Nickel-bearing pyrrhotite is common in I-type but rare in S-type rocks, sedimentary enclaves, restates and country rocks. This eliminates the possibility that the Ohmine S-type rocks were formed by interaction of I-type magma and sedimentary rocks or by mixing between I-type and an ideal S-type magmas, as interpreted from the chemistry of biotite and the chronology of these rocks. On the basis of mineralogy, and isotopic data for S, O, Sr, C, and rare gases, it was shown that I-type granitic rocks were derived from partial melting of intermediate igneous rocks containing hornblende and plagioclase and S-type, from partial melting of sedimentary rocks containing biotite and orthoclase.

Metallogeny and tectonic development of the Tasman Fold Belt System in New South Wales

The northwestern corner of New South Wales consists of the paratectonic Late Proterozoic to Early Cambrian Adelaide Fold Belt and older rocks, which represent basement inliers in this fold belt. The rest of the state is built by the composite Late Proterozoic to Triassic Tasman Fold Belt System or Tasmanides. In New South Wales the Tasman Fold Belt System includes three fold belts: (1) the Late Proterozoic to Early Palaeozoic Kanmantoo Fold Belt; (2) the Early to Middle Palaeozoic Lachlan Fold Belt; and (3) the Early Palaeozoic to Triassic New England Fold Belt. The Late Palaeozoic to Triassic Sydney—Bowen Basin represents the foredeep of the New England Fold Belt. The Tasmanides developed in an active plate margin setting through the interaction of East Gondwanaland with the Ur-(Precambrian) and Palaeo-Pacific plates. The Tasmanides are characterized by a polyphase terrane accretion history: during the Late Proterozoic to Triassic the Tasmanides experienced three major episodes of terrane dispersal (Late Proterozoic—Cambrian, Silurian—Devonian, and Late Carboniferous—Permian) and six terrane accretionary events (Cambrian—Ordovician, Late Ordovician—Early Silurian, Middle Devonian, Carboniferous, Middle-Late Permian, and Triassic). The individual fold belts resulted from one or more accretionary events. The Kanmantoo Fold Belt has a very restricted range of mineralization and is characterized by stratabound copper deposits, whereas the Lachlan and New England Fold Belts have a great variety of metallogenic environments associated with both accretionary and dispersive tectonic episodes. The earliest deposits in the Lachlan Fold Belt are stratabound Cu and Mn deposits of Cambro-Ordovician age. In the Ordovician Cu deposits were formed in a volcanic are. In the Silurian porphyry CuAu deposits were formed during the late stages of development of the same volcanic are. Post-accretionary porphyry CuAu deposits were emplaced in the Early Devonian on the sites of the accreted volcanic arc. In the Middle to Late Silurian and Early Devonian a large number of base metal deposits originated as a result of rifting and felsic volcanism. In the Silurian and Early Devonian numerous SnW, Mo and base metal—Au granitoid related deposits were formed. A younger group of MoW and Sn deposits resulted from Early—Middle Carboniferous granitic plutonism in the eastern part of the Lachlan Fold Belt. In the Middle Devonian epithermal Au was associated with rifting and bimodal volcanism in the extreme eastern part of the Lachlan Fold Belt. In the New England Fold Belt pre-accretionary deposits comprise stratabound Cu and Mn deposits (pre-Early Devonian): stratabound Cu and Mn and ?exhalite Au deposits (Late Devonian to Early Carboniferous); and stratabound Cu, exhalite Au, and quartz—magnetite (?Late Carboniferous). S-type magmatism in the Late Carboniferous—Early Permian was responsible for vein Sn and possibly AuAsAgSb deposits. Volcanogenic base metals, when compared with the Lachlan Fold Belt, are only poorly represented, and were formed in the Early Permian. The metallogenesis of the New England Fold Belt is dominated by granitoid-related mineralization of Middle Permian to Triassic age, including SnW, MoW, and AuAgAs Sb deposits. Also in the Middle Permian epithermal AuAg mineralization was developed. During the above period of post-orogenic magmatism sizeable metahydrothermal SbAu(W) and Au deposits were emplaced in major fracture and shear zones in central and eastern New England. The occurrence of antimony provides an additional distinguishing factor between the New England and Lachlan Fold Belts. In the New England Fold Belt antimony deposits are abundant whereas they are rare in the Lachlan Fold Belt. This may suggest fundamental crustal differences.

紀伊半島中部，大峰地域の中新世ＩタイプおよびＳタイプ花こう岩質岩の微量化学組成

The Ohmine granitic rocks of Miocene age occur in the central Kii peninsula, Outer Zone of Southwest Japan. These rocks fall into two distinct types; S-type and I-type (Dorogawa and Shirakura rocks). Trace elements (Ba, Ce, Co, Cr, Cs, Nb, Ni, Rb, Sc, Sr, Y, Zr, and F) of the rocks were determined by photon activation, colorimetry, and ion-electrode analyses. The S-type granitic rocks are high in SiO2, F, and Rb/Sr and low in Sr compared with the I-type ones. The Shirakura rocks formed early stage in the intrusive sequence are higher in D. I., Ce, Rb, and Nb and lower in MgO/(MgO+FeOt), Co, Cr, Ni, and Sr than the Dorogawa ones. The S-type magma, from which biotite, plagioclase, and orthoclase must have fractionated, would be formed by partial melting of biotite and orthoclase containing sedimentary and/or metasedimentary rocks at about 700°C and 5 kbar. The I-type granitic rocks were probably generated by partial melting of Caamphibole and plagioclase containing intermediate igneous and/or metaigneous rocks. Fractional crystallization mainly of plagioclase, Ca-amphibole, and biotite must have occurred in the I-type magma. The Shirakura magma would be produced by lower degree of partial melting than the Dorogawa one. The crystallization course of the I-type magma was different from that of the S-type one ; orthoclase precipitated next to quartz in the I-type magma, on the contrary quartz crystallized after orthoclase in the S-type magma. On the basis of trace elements behavior, it is unlikely that the granitic rocks are products of large scale magma-crust interaction and of mixing between basaltic and granitic magmas. On the criteria of alumina saturation and isotope ratio, revised classification of calc-alkali granitic rocks are proposed; C-type (sub to metaluminous, SrIo〉 0.708) and W-type (per. aluminous, SrIo〈0.708) in addition to I-type and S-type. C-type rocks are, however, not found yet. The parental materials and fractionating phases of the I-type, W-type, and the S-type in the world are estimated from publicated data. The estimated parental materials and fractionating phases of the I- and W-type and the S-type are the same as those of the Ohmine I-type and the Ohmine S-type, respectively. The I- and W-type rocks are, therefore, in lower range of Rb/Sr than the S-type ones. Many I- and W-type granitic rocks occur in mature continental regions composed of basaltic and granitic layers. The S-type granitic rocks would, on the other hand, occur in the regions consisting of immature crust without basaltic layer (e. g. southern part of the Outer Zone of Southwest Japan), and in the regions of crust exceeding 50km in thickness such as continental margins (e. g. western Bolivia) and continent-continent collision zone (e. g. High Himalaya).

紀伊半島中部，大峰地域の中新世ＩタイプおよびＳタイプ花こう岩質岩の岩石学

The Ohmine granitic rocks of Miocene age (11 to 16 Ma) occur in the central Kii peninsula, Southwest Japan. These rocks fall into two distinct types: I-type granitic rocks (11.6 to 14.2 Ma) and S-type ones (12.6 to 15.6 Ma). Mineral assemblage, major geochemical and isotopic criteria provide the discriminant between the two types. The Ohmine I-type rocks have lower Sr initial ratios (0.7049 to 0.7058), δ18O values (7.2 to 7.6 ‰) and heavier δ34S values (-2.7 to -3.6‰) than the S-type rocks (0.7083 to 0.7086, 10.5 to 11.5‰ and -16.2‰, respectively). Chemical compositions of biotite and pyrrhotite from both type rocks are also different. Biotites from the I-type rocks have high Si and low Al as compared with those from the S-type rocks. Ni-detectable pyrrhotite is frequently present in the I-type rocks but is absent in the S-type ones with only one exception. The S-type granitic rocks, having non-minimum melting compositions in the granite and granodiorite systems, contain mineral clots such as biotite+sillimanite+quartz+orthoclase+cordierite+garnet and biotite+garnet+quartz+orthoclase+cordierite+orthopyroxene, occasionally within quartz and plagioclase phenocrysts. Their mineral assemblages and Fe-Mg distribution for garnet-cordierite and garnet-biotite pairs indicate the conditions of granulite facies (5kb and 700°C). Neither sillimanite (including fibrolite) nor garnet is present in the metamorphosed country rocks and sedimentary enclaves. The S-type magma is probably generated by partial melting of sedimentary rocks under the conditions of not more than 5kb and 700°C, and the mineral clots can be regarded as refractory residues (restites). On the other hand, the I-type magma is considered to be produced by partial melting of metamorphosed igneous rocks. In the Outer Zone of Southwest Japan, the I-type granitic rocks occur on the northern side of the Butsuzo Tectonic Line and the S-type ones on the southern side. The distribution of both the type rocks reflects the differences of source materials at the lower crust (at a depth of about 20km). The lateral change of the lower crustal materials in the N-S direction is also confirmed on the basis of the seismic wave velocities.