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S-type Granites (s-type + granite)
Selected AbstractsThe Cornubian Batholith: an Example of Magmatic Fractionation on a Crustal ScaleRESOURCE GEOLOGY, Issue 3 2006Bruce W. Chappell Abstract. The Cornubian Batholith comprises six major and several smaller bodies of S-type granite in southwestern England. These late-Variscan granites comprise two-mica granites, and much less abundant Li-mica granites that are restricted to one of the major bodies (St Austell) and smaller bodies. Some of these intrusive rocks are associated with major Sn mineralization. This paper is concerned with the geochemistry of the two-mica granites, which are felsic, strongly peraluminous, and have a high total alkali content and low Na:K. Rocks with very similar compositions to these granites occur elsewhere, including the Variscan granites of continental Europe, and in southeastern Australia. In detail all of the major plutons of this batholith have distinctive compositions, except for Bodmin Moor and Carnmenellis which cannot be discriminated from each other compositionally. A comparison with experimental data shows that the granites attained their major element composition under conditions of crystal-liquid equilibrium, with the final melt being saturated in H2O, at temperatures close to 770d,C and pressures about 50 MPa. That temperature estimate is in good agreement with values obtained from zircon saturation thermometry. The specific minimum-temperature composition excludes the possibility of widespread transfer of elements during hydrothermal alteration. Minor elements that are relatively very abundant are Li, B, Cs and U, while F, Ga, Ge, Rb, Sn, Ta, W and Tl are quite abundant and P is high for felsic rocks. Sr, Ba, and the trace transition metals Sc to Zn, are low, but not as low as they commonly are in very felsic granites. These trace element abundances, and the EL2O-saturation, resulted from the fractional crystallization of a melt derived by the partial melting of feldspathic greywackes in the crust. The Cornubian granites have compositions very similar to the more felsic rocks of the Koetong Suite of southeastern Australia, where a full range of granites formed at the various stages of magmatic fractionation postulated for the Cornubian granites, can be observed. The operation of fractional crystallization in the Cornubian granites is confirmed by the high P abundances in the feldspars, with P contents of the plagioclase crystals correlating with Ab-con-tent Most of the granites represent solidified melt compositions but within the Dartmoor pluton there is a significant component of granites that are cumulative, shown by their higher Ca contents. The Cornubian plutons define areas of high heat flow, of a magnitude which requires that fractionated magmas were transported laterally from their sources and concentrated in the exposed plutons. The generation of these granite plutons therefore involved magmatic fractionation during the stages of partial melting, removal of unmelted material from that melt, and fractional crystallization. During the later stages of those processes, movement of those magmas occurred on a crustal scale. [source] Timing and nature of fluid flow and alteration during Mesoproterozoic shear zone formation, Olary Domain, South AustraliaJOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2005C. CLARK Abstract The development of shear zones at mid-crustal levels in the Proterozoic Willyama Supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration. Monazite dating of shear zone fabrics reveal that they formed at 1582 ± 22 Ma, at the end of the Olarian D3 deformational event and immediately prior to the emplacement of regional S-type granites. Two stages of fluid flow are identified in the area: first an albitizing event which involved the addition of Na and loss of Si, K and Fe; and a second phase of calcsilicate alteration with additions of Ca, Fe, Mg and Si and removal of Na. Fluid fluxes calculated for albitization and calcsilicate alteration were 5.56 × 109 to 1.02 × 1010 mol m,2 and 2.57 × 108,5.20 × 109 mol m,2 respectively. These fluxes are consistent with estimates for fluid flow through mid-crustal shear zones in other terranes. The fluids associated with shearing and alteration are calculated to have ,18O and ,D values ranging between +8 and +11,, and ,33 and ,42,, respectively, and ,Nd values between ,2.24 and ,8.11. Our results indicate that fluids were derived from metamorphic dehydration of the Willyama Supergroup metasediments. Fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions. Shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent ,retrograde' fluid-flow event. Thus, the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times, prior to regional granite formation, during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing. [source] Geochemistry of K-feldspar and Muscovite in Rare-element Pegmatites and Granites from the Totoral Pegmatite Field, San Luis, ArgentinaRESOURCE GEOLOGY, Issue 4 2009Julio Oyarzábal Abstract The geochemistry of K-feldspar for K, P, Sr, Ba, Rb, Cs, Ga, and of muscovite for the same elements plus Nb and Ta, was used for proving the parental relationships of S-type granites and LCT (Li, Cs, Ta) rare-element pegmatites in the southernmost pegmatitic field of the Pampean pegmatite province in Argentina. The variation of K/Rb-Cs, K/Cs-Rb, K/Rb-Rb/Sr, K/Rb-Ba in K-feldspar from the granites and pegmatites show that they form an association with the evolutional sequence: granites , barren- to transitional pegmatites , beryl type, beryl-columbite-phosphate pegmatites , complex type of spodumene subtype pegmatites , albite-spodumene type , albite type pegmatites. This sequence reflects the regional distribution of the different magmatic units. The Ta-Cs diagram for muscovite reveals that none of the studied pegmatites exceed the threshold established in previous studies for being considered with important tantalum oxide mineralization. The granites and pegmatites constitute a rare-element pegmatitic field in which different magmatic units form a continuous fractionation trend, extended from the less evolved granitic facies to the most geochemically specialized pegmatites [source] Helium Isotope Geochemistry of Ore-forming Fluids from Furong Tin Orefield in Hunan Province, ChinaRESOURCE GEOLOGY, Issue 1 2006Zhao-li Li Abstract. The Furong tin orefield, located in southern Hunan, China, is a newly-discovered super-large tin orefield. In contrast to most other tin deposits associated with S-type granites, the Furong tin deposit is closely associated with the Qitianling A-type granite. The 3He/4He ratios of fluid inclusions in pyrite and arsenopyrite from this orefield range from 0.13 to 2.95 Ra. The influence of various post-mineralization processes on the helium isotopic composition of ore-forming fluid inclusions are estimated negligible. Thus, the analytical values of helium isotopic composition basically represent the original values of ore-forming fluids at the time they were trapped. The 3He/4He ratios of ore-forming fluids from the Furong orefield indicate the existence of mantle-source components. It supports the idea that both the Furong tin orefield and Qitianling granite formed under the geodynamic background of mantle upwelling and crustal extension. [source] | ||||||||||