Late Magmatism of the Galway Granite Batholith: II. Composite Dolerite–Rhyolite Dikes
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Composite dolerite-rhyolite dikes traverse the Galway Granite batholith and its adjacent envelope. The dikes pertain to the Teach D6ite suite and were previously considered to be of Carboniferous age. New and extended examination of field relationships supports recent radiometric dating for an intrusive period that overlapped with the final consolidation of the Galway batholith. Regional crustal extension produced a complex pattern of fissuring, controlled by various preexisting structures, which permitted ascent of mantle-derived melts into and around the Galway batholith. Ponding of mafic magma at an intermediate level facilitated crustal partial melting and the generation of high-silica, high-alumina rhyolitic melts. The two contrasting magmas then rose into common or proximate dike fissures, rhyolitic injection immediately following that of dolerite. Magma storage in stratified chambers occasionally resulted in the development of a hybrid magma layer, but in all cases minor mingling and mixing beween dolerite and rhyolite magma continued up into the dikes. Rhyolite geochemistry precludes a genetic relationship with the Galway granitoids, despite a few instances where granitic material was entrained into rhyolitic magma. Introduction and setting The 400Ma Galway Granite batholith was emplaced into 470Ma island-arc orthogneisses in the Connemara sector of the Caledonides. This emplacement was followed by the intrusion of two hypabyssal suites: earlier microphyric ('porphyry') dacite dikes (Kinahan 1869; Mohr 2003) and a later complex nexus of dolerite dikes, the Teach D6ite (TD) suite (Mitchell and Mohr 1987; Fig. 1). The numerous and widespread dacite dikes have consistently been considered the youngest igneous rocks pertaining to the Galway batholith (Wager 1932; Wright 1961; Harvey 1967; Coats and Wilson 1971; Senior 1973; Leake 1974). However, new work summarised here suggests that the subsequent TD dikes were a final manifestation of the magmatic episode responsible for the batholith. The regional pattern of TD dikes comprises three major linear trends (Fig. 1). In the central part of the Galway batholith and its northern envelope, the NNE-trending Seanabhain system of dikes is intimately associated with the Shanawon Fault that separates the central and western blocks of the batholith (Feely and Madden 1988; Mohr 1993; Callaghan 1999). The na hUillinni dike system, 3.5km west of the Seanabhan system and parallel to it, projects much farther NNE into the orthogneiss envelope (Fig. 1). Secondly, a grid of ENE-trending dikes Irish Journal of Earth Sciences 22 (2004), 15-32. © Royal Irish Academy 15 This content downloaded from 207.46.13.174 on Sun, 10 Jul 2016 05:04:50 UTC All use subject to http://about.jstor.org/terms 16 Irish Journal of Earth Sciences (2004)Keywords:
Batholith
Dike
Dacite
Sill
Magma chamber
Porphyritic
Vergara and Munizaga (1974) reported an age of 760,000 years for a sample of rhyolite-dacite obsidian from the mainly late Oligocene to Miocene Central Valley volcanic belt at Ancud, Chiloe (Fig. 1). They interpreted this sample as representing “a last and isolated volcanic event in this series”.
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The Ermiaogou Cu-Au deposit is located within the Zijinshan ore field in southwestern Fujian Province. This deposit has similar styles of mineralization and alteration with the Zijinshan Cu-Au deposit. Mineralization was surrounded by acidic subvolcanic and closely related to porphyritic dacites and breccias,which suggested that both deposits may share similar genesis. Both the Ermiaogou Cu-Au deposit and the Zijinshan Cu-Au deposit belong to high-sulfidation epithermal low temperature hydrothermal deposits, which are significantly different from the Luoboling porphyry Cu-Mo deposit and the Yueyang low-sulfidation epithermal low temperature hydrothermal deposit. The Ermiaogou porphyritic dacites are closely related to the Cu-Au mineralization within the deposit. Our new precise zircon U-Pb dating results reveal that the Ermiaogou porphyritic dacite was emplaced in the Early Cretaceous( 106 ~ 108Ma), which are similar to those of porphyritic dacite in the Zijinshan deposit. These results suggest that both deposits were resulted from the same period of magmatism and the large-scale volcanic-intrusive activity related to mineralization in the Cretaceous. Geochemically,the Ermiaogou porphyritic dacite shows high Si and K contents and weakly peraluminous. The rocks are enriched in LILEs and LREE; depleted in Nb,Ta,P,Ti; no / weak negative Eu; and characterized by strongly fractionated REE pattern and high Sr / Y and La / Yb ratios. It also exhibits low initial87Sr /86Sr ratios( 0. 712698 to 0. 713174),moderate e Nd( t)(- 6. 26 to- 4. 94) and high zircon eHf( t)(-4. 52 to 1. 66) values. Interpretation of the elemental and isotopic data suggests that the Ermiaogou porphyritic dacite has some affinities with the adakite,and was most likely generated via a process including mixing of enriched mantle-derived mafic magmas and induced felsic magmas formed by partial melting of lower crust materials under a back-arc extensional environment that may have caused by the subduction of the Paleo-Pacific plate. Enriched mantle-derived melts have high oxygen fugacities and high H 2 O content, thus potentially are major sources for Cu-Au metals. Mafic magmas derived from enriched mantle can change their hydrous and redox state by mixing with crust-derived melts,and led to generate adakitic magma by magmatic differentiation under high pressure,which may further result in the relevant Cu-Au mineralization at Ermiaogou.
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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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SHRIMP Zicon U-Pb Dating and Its Geological Implication for the Xiangshan Porphyric Dacite-Rhyolitic
The Xiangshan volcanic complexes is located at the intersection of two fault systems:the first one,with northeast strike,is named the Yongfeng-Fuzhou fault and the other,with nearly north-south strike,is called the Yihuang-Ningdu fault.The volcanic eruption can be divided into two events.The lower part of the first event was dominated by volcanic sediments,with a 70 m thick interlayer of tuff lava,while the major component of the upper part was porphyritic dacite-rhyolite.Distinct from the first event,the second one was produced a rhyolite-tuff with volcanic sedimentary laminas at bottom and porphyroclastic rhyolite lavas on top.Previous reports have provided many dates for these strata,such as 140.3 Ma for the porphyroclastic rhyolite lavas in the second event and the 136.0 Ma for the porphyritic dacite-rhyolite in the first event.However there is some conflict due to the low accuracy of the data,for the porphyroclastic rhyolite lavas should be younger than the porphyritic dacite-rhyolite.In this paper,the SHRIMP zircon U-Pb dating technique has been applied to the porphyritic dacite-rhyolite in the first event of Xiangshan volcanic complexes,and we obtain an 206Pb/238U age 136.6±2.7 Ma(MSWD=1.8).Then we can confirm that the eruption time of the porphyritic dacite-rhyolite should be about 136.6 Ma.Combined with the former reported ages,it seems more reasonable that the actual ages of the porphyroclastic rhyolite lavas should be a little younger than that formerly reported.
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Hydration of obsidian has been used to date rhyolite flows, containing obsidian or porphyritic glass, at Glass Mountain (Medicine Lake Highlands) and Mono Lake, California. The method is simple and rapid and can be used to date flows that erupted between 200 and approximately 200,000 years ago.
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The Sandaowan rhyolite-dacite porphyry,located to the west of the Golmud River,about 55 km from south of Golmud City,is an intrusive vein of volcanic craters.It intruded into the clastic rocks,named Habalayigou group as one of the Nachitai unit in this region.A sample No.B727-1 was collected from the Sandaowan rhyolite-dacite porphyry rock.We used the LA-MC-ICPMS method of Zircon U-Pb Dating in this article.The 25 zircons from No.B727-1 yield a mean value of(425.9 ± 2.6) Ma of 206Pb/238U,which is interpreted as the age of the volcanism.The result shows that the volcanic crater was one of the eruptive center during Late collision of the Early Paleozoic in this region.Besides,it reveals a low time restrict to the Habalayigou group,implying that the Habalayigou strata formed before the Middle Silurian.These results conflict with previous 40Ar /39Ar whole rock ages of 147.96Ma for the rhyolite-dacite porphyry from the same rock.
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금당도지역에 분포하는 화성암은 구성광물, 야외증거와 산상을 근거로 유문암, 반정질유문암, 중성맥암으로 구분되어 진다. TAS(total alkali-silica) 다이아그램에서 유문암은 유문암-데사이트의 영역에 그리고 반정질유문암은 유문암 영역에 속한다. 유문암과 반정질유문암의 생성시기를 밝히기 위하여 전암을 이용한 K-Ar법으로 절대연대측정이 수행되었으며 그 결과 전자는 76-78Ma로 백악기말 캄파니안에, 후자는 71-72Ma로 캄파니안과 마스트리크티안의 경계에 속한다. 이 지질연대는 백악기말 동안 한반도 남부지역에서 활발하게 진행되었던 유천층군의 화성활동에 대비된다. 한반도 서남부지역에 분포하는 화산암과 백악기층에서 발견되는 다양한 시기와 성분을 갖는 화산암의 지질연대를 근거로 이 지역에서 108-71Ma 사이에 유라시아판과 태평양판의 충돌에 의한 활발한 화성활동이 지속적으로 진행되었을 지시한다. Based on mineral assemblages, field occurrences, the volcanic rocks distributed in the Geumdang Island area are divided into three types: rhyolite, porphyritic rhyolite and intermediated dyke rock. In a diagram of [TAS (total alkali-silica)], rhyolites and porphyritic rhyolites belong to the rhyolite-dacite field and rhyolite field, respectively. As to the times when the rhyolite and porphyritic rhyolite rocks were formed a whole rock K-Ar age was obtained. These absolute age determinations have revealed that the former (rhyolite) has an age of 76-78 Ma and belongs to the Late Cretaceous (Campanian) and the latter (porphyritic rhyolite) is 71-72 Ma in age and thus belongs to the boundary between the Campanian and Maastrichtian. These geological ages are associated with the igneous activity of the Yuchon Group which occurred vigorously in the southern part of the Korean peninsula during the Late Cretaceous. The various geological ages of volcanic rocks distributed in the southwestern part of the peninsula and of igneous rocks found in the Cretaceous formation which contain a wide variety of minerals indicate that in this area, volcanic activities continued vigorously as a result of the collision of the Eurasian and Pacific Plates between 108-71 Ma.
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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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The chemical and isotopic compositions of sills at West Mageik Lake are contrasted with high‐silica rhyolite erupted in 1912 at the nearby Valley of Ten Thousand Smokes to gain insight into the petrogenesis of evolved magmas in the Aleutian arc. A high‐silica rhyolitic sill has an ε Nd value (+8.8) similar to an adjacent dacite porphyry stock, but the elevated Al, Sr, and 87 Sr/ 86 Sr of the rhyolite suggest that fractionation of a parental dacite magma was accompanied by significant contamination by Sr‐rich sediments, probably of Paleozoic age. The chemical and isotopic character of the West Mageik Lake rhyolite suggest that the system was of small volume, making it easily affected by crustal contamination. The fact that the 8 km³ of high‐silica rhyolite erupted at the Valley of Ten Thousand Smokes shows only a slight Sr isotopic shift from associated dacite and andesite suggests it was part of a larger system that could assimilate small masses of crust without having its composition significantly affected.
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