U-Pb DATING OF HYDROTHERMAL TITANITE RESOLVES MULTIPLE PHASES OF PROPYLITIC ALTERATION IN THE OYU TOLGOI PORPHYRY DISTRICT, MONGOLIA
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Abstract Oyu Tolgoi is a world-class, Late Devonian porphyry district in southern Mongolia. Because of its age and geodynamic setting, it has undergone a complex geological history that includes major postmineralization magmatic-hydrothermal events in close proximity to the porphyry deposits. The propylitic alteration halos that surround the Cu-Au deposits contain widespread hydrothermal titanite, as do the younger altered volcanic and intrusive rocks. Here, we present a comprehensive laser ablation-inductively coupled plasma-mass spectrometry U-Pb study on in situ, propylitic titanite from the Oyu Tolgoi district. The results identify district-wide hydrothermal alteration episodes that coincide with known magmatic events: Devonian porphyry mineralization (~372 Ma); the intrusion of granodiorite plutons and andesite dikes in the Carboniferous (~320–310 Ma); and the emplacement of the Permian Khanbogd Granite alkaline batholith (~290 Ma). Both Carboniferous and Permian alteration events variably overprint the earlier porphyry alteration halo. Overdispersion in the U-Pb data from some samples, due to Pb and/or U mobility, makes interpretation of some titanite ages more difficult, but further exemplifies the complex alteration history of the district. We conclude that U-Pb dating of propylitic titanite is a viable means by which explorers can identify alteration within a prospect that is synchronous with potentially fertile intrusions of known age. The extent of the coeval propylitic alteration and its mineral chemistry can then be used to assess the likelihood of a major porphyry center being present.Keywords:
Titanite
Porphyritic
Geochronology
Batholith
Devonian
The intrusion of the Cassia batholith in Idaho was accompanied by clearly denned contact phenomena expressed by the impregnation of "igneous" material into quartzite, and in such a manner as to make it resemble granitic rock. While the magma crystallized as granodiorite, it sent forth emanations into the invaded rock and by progressive replacements produced therein a broad zone of porphyritic granite. As a result of the progressive granitization of the quartzites in the contact zone, the batholith is now about three times as large as it would have been had "replacement" not been a factor in its intrusion. It is altogether likely that the changes noted in the contact zone of the Cassia batholith may also be expressed in the crystalline schists in extensive metamorphic regions. The replacement origin of the microcline phenocrysts in the hybrid porphyritic granite also directs attention to the probability that the microcline phenocrysts in normal granitic rocks also may have been formed late by replacement.
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The late-Hercynian magmatic alignement of the Los Pedroches Batholith in South Central-Iberian Zone (Iberian Massif) exhibits a conspicuous dike swarm. Dikes are in chronological order: a) traquiandesite, b) dacite to rhyodacite, c) rhyolite, d) aplite, aplopegmatite and pegmatite, e) quartz, f) basic (diabase, lamprophyre).
Rhyolite dikes (granite to adamellite) form a number of lineal swarms oriented N120-130E. The main group extends almost undisturbed from near Belalcazar (Cordoba) to the Guadalquivir fault (Jaen). This array, ca. 130 km length and 2-12 km thick, is usually composed of 3 to 50 dikes cutting across the main plutonic facies of the Batholith.
Rhyolite dikes have the common granitic minerals, including zircon, apatite and ilmenite as accessories. According to their macroscopic features and petrography the dikes may be porphyritic microgranite, porphyritic rhyolite or porphyritic granophyre, whereas according to the mineral cheroistry the dikes range froro rhyolite-granite to alkali feldspar granite-rhyolite. The cheroical composition of studied rocks corresponds to peraluminous and calc-alkaline
terms of a K-rich alumino-cafemic calc-alkaline association.
The dike swarm cuts granite massifs of the Los Pedroches batholith intruded at ca. 300 ± 6 Ma (El Guijo pluton, Fernandez et al., 1990), but the emplacement is poorly constrained as inferred by K-Ar mineral ages of ca. 315 ±15 Ma (Bellon et al, 1979) and Rb-Sr whole rockage of 295 ± 18 Ma (Defalque et al., 1992). The dike swarm would result of subvolcanic intrusions genetically related to the late-Hercynian igneous activity that originated the Los Pedroches Batholith.
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1973)
Many andesitic lava flows of the calc-alkaline rock series erupted during the middle to late Miocene time are distributed in the western part of the Setouchi Inland Sea volcanic province. They cna be divided into the following three stages: 1) glassy andesitic and biotite dacitic, dykes 2) hornblende bearing andesitic laves and their pyroclastics, and 3) andesites forming domes or lavas and their pyroclastics. Porphyritic andesites are characterized by high MgO content, though calculated groundmass compositions as well as aphyric andesites reveal the nature of the common Japanese calc-alkaine series. MgO-FeO+Fe2O3-Na2O+K2O diagram shows that these andesitic rocks are chemically divided into MgO-poor and MgO-poor and MgO-rich groups. The former group carries granitic xenolith, indicating the effect of contamination.
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Abstract This paper investigates the reasons for the presence of both domed inselbergs (bornhardts) and boulder inselbergs (koppies) on the Matopos Batholith, Zimbabwe. XRF and microprobe analyses of rock samples from 16 bornhardts and 24 koppies have shown no significant chemical or mineralogical differences which might indicate greater weathering resistance of the bornhardts. Many bornhardts are located on massive bodies of slowly‐cooled porphyritic granite which were emplaced at a relatively late stage in the evolution of the batholith. The surrounding rocks are less porphyritic and appear to have been foliated during deformation associated with emplacement of the later bodies. The entire batholith has been affected by post‐emplacement tectonism which has created three major fracture systems. The morphological differences between bornhardts and koppies in this area are therefore considered to reflect differences in jointing which arise both from the primary igneous emplacement process and from tectonic deformation.
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The Precambrian Cachoeirinha-Salgueiro Fold Belt (CSF) located in the western portion of the states of Pernambuco and Paraiba is intruded, in its northern portion, by several coarsely porphyritic potassic calc-alkalic .batholiths. These batholiths were syntectonicaly unplaced in relation to the Brasiliano cycle (=Pan-African) and are commonly associated with potassium diorites suggesting coexistence and mixing between felsic and mafic magmas. In the Itaporanga batholith three petrographic domains-were mapped. A hybrid zone characterized by intense mechanical mixing of granite to granodiorite and potassium diorite magmas is located towards the border of the batholith. A commingling zone where felsic porphyritic granite to granodiorite and potassium diorite rocks are individualized at outcrop scale is located towards the center of the batholith. Finally a felsic porphyritic fades occur in the hybrid zone. The intense mechanical mixing observed in the hybrid zone developed migmatite-like structures (stromatic and less frequently agmatic), pillov -like structures, mafic enclaves with irregular shape and cuspate contacts between mafic and felsic rocks, suggesting diffusion of chemical species across contact. Major and trace element plotted against SiO 2 agree with a mixing model to explain the hybrid samples plotted at intermediate position between felsic and mafic rocks. Similarity among chemical analyses of amphiboles from potassium dioritic enclaves of the Itaporanga batholith and from the potassium diorite stock east of it suggest a common source for both magmas. This hypothesis is corroborated by similar REE patterns for potassium dioritic enclaves of the Itaporanga batholith and for the potassium diorite stock. The batholith shows a well developed foliation which dips towards its core suggesting that the present level of exposure represents the root zone of a diapir, where intense interaction between felsic and mafic magmas took place.
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The Precambrian Cachoeirinha-Salgueiro Fold Belt (CSF) located in the western portion of the states of Pernambuco and Paraiba is intruded, in its northern portion, by several coarsely porphyritic potassic calc-alkalic .batholiths. These batholiths were syntectonicaly unplaced in relation to the Brasiliano cycle (=Pan-African) and are commonly associated with potassium diorites suggesting coexistence and mixing between felsic and mafic magmas. In the Itaporanga batholith three petrographic domains-were mapped. A hybrid zone characterized by intense mechanical mixing of granite to granodiorite and potassium diorite magmas is located towards the border of the batholith. A commingling zone where felsic porphyritic granite to granodiorite and potassium diorite rocks are individualized at outcrop scale is located towards the center of the batholith. Finally a felsic porphyritic fades occur in the hybrid zone. The intense mechanical mixing observed in the hybrid zone developed migmatite-like structures (stromatic and less frequently agmatic), pillov -like structures, mafic enclaves with irregular shape and cuspate contacts between mafic and felsic rocks, suggesting diffusion of chemical species across contact. Major and trace element plotted against SiO2 agree with a mixing model to explain the hybrid samples plotted at intermediate position between felsic and mafic rocks. Similarity among chemical analyses of amphiboles from potassium dioritic enclaves of the Itaporanga batholith and from the potassium diorite stock east of it suggest a common source for both magmas. This hypothesis is corroborated by similar REE patterns for potassium dioritic enclaves of the Itaporanga batholith and for the potassium diorite stock. The batholiths shows a well developed foliation which dips towards its core suggesting that the present level of exposure represents the root zone of a diapir, where intense interaction between felsic and mafic magmas took place.
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Abstract Park Volcanics Group is proposed for igneous rocks, either shallow intrusive or extrusive, emplaced in the Murihiku Terrane during Triassic‐Jurassic times. The term replaces Park Intrusives of Mutch, some members of which are shown to be extrusive rather than intrusive. Formation status within the group is given to Gowan Andesite and Pinney Volcanics (new names) in western Southland, Glenham Porphyry in eastern Southland, and Barnicoat Andesite (new) in the Richmond area, Nelson. Gowan Andesite is a porphyritic feldspar two‐pyroxene andesite with a glassy or microcrystalline groundmass. A suite of low‐grade metavolcanic rocks which forms the main mass of Malakoff Hill and which has formerly been included in the "Park Intrusives" is here excluded and ascribed to the Takitimu Group; representative chemical data are given. Glenham Porphyry is typically a porphyritic feldspar two‐pyroxene andesite texturally similar to the Gowan Andesite but with significant geochemical differences. Two volumetrically minor members are recognised, Habukinini Trachydacite and Kenilworth Rhyolite. In the north of its outcrop area, Glenham Porphyry is emplaced on or into Late Triassic terrestrial beds; in the middle it overlies Kaihikuan (Middle Triassic) and is overlain by Otapirian (latest Triassic) marine beds; and in the southeast it is directly overlain by Ururoan (late Early to early Middle Jurassic) conglomerates and marine sandstones. Pinney Volcanics are restricted to a very few, probably one, massive conglomeratic horizon in the Oretian Stage. The commonest rock type is a two‐pyroxene trachydacite, modified by very‐low‐grade burial metamorphism. Auto‐brecciation is characteristic and rock types change over short distances. Hornblende‐rich variants occur as well as more felsic varieties including rhyolite ignimbrite. These may have been erupted onto a bouldery floodplain or shallow‐marine surface, but alternatively may have been mass‐emplaced by debris avalanche resulting from flank collapse of a volcano lying beyond the present terrane boundary. In its unaltered condition, Barnicoat Andesite was an olivine two‐pyroxene andesite, distinctly more tholeiitic than the Gowan and Glenham andesites. A previously reported occurrence of "Park Intrusives" in the southern Taringatura Hills is discredited, as is the presence of Jurassic strata in that area. Weetwood Formation, in the eastern flanks of the Takitimu Mountains, is provisionally excluded from the Park Volcanics. Radiometric data indicate a Late Triassic or Early Jurassic age for the Gowan, Glenham, and Barnicoat andesites. Excluding the Pinney Volcanics, no more than 5 unequivocal volcanic events are recognised during more than 80 million years of sedimentation in the Murihiku Basin. A minimum volume of 7.5 km3 is estimated for the Glenham Porphyry and about one‐third of that figure for the Gowan Andesite. The volume of Pinney Volcanics and Barnicoat Andesite appears to be much less. Medium to high K2O in the Park Volcanics suggests that the locus of Murihiku sedimentation was a back‐arc or possibly intra‐arc rather than a fore‐arc basin. Key words: Park Volcanics GroupGowan AndesiteGlenham PorphyryPinney DaciteBarnicoat AndesiteWeetwood FormationMurihiku TerraneTakitimu GroupTriassicJurassicnew stratigraphic names
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