Coexistence and mixing of magmas in the late of cambrian Itaporanga Batholith, State of Paraíba, Northeastern Brazil
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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.Keywords:
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Porphyritic
Diorite
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Abstract Enclaves of diverse origin are present in minor amounts in the coarse-grained biotite granites of the Cornubian batholith, southwest England. The most common enclave type is layered, rich in biotite, cordierite and aluminosilicates, and has textures and compositions that reveal variable degrees of melt extraction from metasedimentary source rocks. Rare sillimanite-bearing enclaves represent residual material, either from the region of magma generation or its ascent path, but most such enclaves were probably derived from the contact aureole closer to the present level of exposure. These non-igneous enclaves (NIE) and their disaggregation products are present in all major plutons, comprising from < 2 to 5 vol.% of the granites. Enclaves of igneous origin are also present in all major plutons except Carnmenellis, generally comprising < 1 vol.% of the granites. The most common type is intermediate in composition, with microgranular texture, and mineral compositions and textures consistent with an origin by magma mixing. Large crystals of K-feldspar, plagioclase and quartz, common in these microgranular enclaves (ME) but absent in NIE, represent phenocrysts derived from the silicic end-member during magma mixing events rather than products of metasomatism as suggested previously. Although the composition of the mafic end-member (basaltic or lamprophyric) involved in the mixing process is poorly constrained, the presence of ME in the granites, and the preponderance of mantle-derived mafic rocks in the coeval Exeter Volcanics, indicate that mafic magma injection into the crust was a factor in the generation of the batholith. Advection of sub-crustal heat provides an explanation for large-volume crustal melting in regions of relatively thin crust such as southwest England.
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Dioritic to quartz monzonitic rocks of the Dartmouth Pluton exhibit excellently preserved, diverse features produced by mingling and mixing of mafic and felsic magma during multiple events. The related mafic and hybridized intermediate composition rocks occur both as discrete outcrop-sized masses or as enclaves within quartz monzonite or early-stage mixed rocks. Enclaves are rounded, lack chilled margins, and in some cases exhibit cuspate margins; they range in size from 1m--<1cm. Outcrops dominated by dioritic rock consist of well developed mafic pillows with inter-pillow infillings of hybridized rock that had been subjected to magma mixing during or prior to the final mingling process. Dioritic rocks are fine-grained with sparse plagioclase phenocrysts; they contain small, darker-colored enclaves indicative of preceding magma interaction. Major and trace element variation diagrams for this suite of rocks exhibit general linear trends consistent with mixing processes. Overall, field, petrographic, and geochemical relationships in the Dartmouth Pluton demonstrate: (1) widespread mingling of mafic and felsic magma, (2) variable degrees of mafic and felsic magma mixing, and (3) multiple and repeated episodes of mafic and felsic magma interaction. Significantly, some spatially associated dioritic and granitic rocks, including a 595 Ma alkali feldspar granite formerly considered to be part ofmore » the Dartmount Pluton, are geochemically related. Field mapping demonstrates that rocks of the mixed suite are intrusive into these rocks, thus establishing a maximum age, but raising the questions that the suite may be considerably younger.« less
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The Late Devonian postorogenic Cobaw batholith, in southeastern Australia, is an oval, east-west-orientated, terrane-stitching lopolith that intruded low-grade metaturbidites. The initial intrusion (at 370 Ma) was the small, hypabyssal, S-type Rainy Creek Rhyolite (RCR). At 369 Ma, the foliated S-type Pyalong pluton was emplaced, apparently along an east-west-orientated fracture zone. Around 367 Ma, the main I-type Baynton pluton intruded as numerous shallow-dipping sheets. The last plutonic event was the intrusion of the broad, thin, flat-lying, and crosscutting sheet of the I-type Beauvallet pluton. The Cobaw plutons had independent origins, with magmas derived from contrasting, internally heterogeneous source rocks. For both the RCR and the Pyalong pluton, the sources were old metasedimentary rocks, while the magmas for the Baynton and Beauvallet plutons formed through partial melting of metadacitic to meta-andesitic rocks, probably with an admixture of immature volcaniclastic graywackes. The source rocks for the Baynton magmas were more crustally evolved than those for the Beauvallet magmas. Heterogeneity in the Baynton pluton did not result from crystal fractionation or magma mixing, despite the relatively high abundance of igneous-textured microgranular enclaves (MEs). The MEs show neither chemical nor isotope mixing trends with each other or with the host magmas. Variations in the Baynton magmas were derived from the heterogeneity of the source terrane, with individual magma batches formed from mixtures of metaigneous and metasedimentary rocks. Baynton MEs are isotopically less evolved than their host rocks. Their chemistry, textures, and field relations suggest that they represent small volumes of hybrid magmas formed through near-source mixing between crustal melts and mantle-derived mafic magmas. Further modification occurred through melt loss and ingestion of host crystals as the MEs were deformed, in the plastic state, during magmatic flow of their hosts.
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Abstract The Toro Complex is one of the Pan-African Older Granites of Nigeria, first described as a reversely zoned pluton made of a central dioritic mass surrounded by a broad granitic rim. It has been thoroughly reinvestigated both from the petrographic and structural points of view, with the help of systematic anisotropy of magnetic susceptibility (AMS) measurements. The granite main body is a hornblende–biotite porphyritic monzogranite characterized by an early submagmatic fabric displaying a concentric pattern of foliations and west plunging lineations (stage 1). This fabric is overprinted by a later one due to solid-state strain along north-south subvertical dextral shear zones (stage 2). In the vicinity of the diorite, an evengrained granite displays magmatic structures that are contemporaneous with this strike-slip event. The diorite–granite contact is a complex zone where field, petrographic and geochemical data enable recognition of the effects of mixing and mingling between a mafic and a felsic magma. Tonalites cropping out within this contact zone are interpreted as hybrid rocks. The reverse zonation of the diorite itself is also the result of some hybridization process. Magmatic interactions mainly resulted from in situ infiltration of granitic liquid into the dioritic mass. The detailed history of this bimodal intrusion began with the emplacement of the granitic magma acquiring a first stage fabric. Before full crystallization of the granitic core, intrusion of the dioritic magma permitted reheating of the granitic magma that then crystallized with specific structural characters. The second stage structures, whether characterized by magmatic fabric near the diorite or by solid-state strain features in north–south shear zones elsewhere in the granite, are related to late Pan-African dextral strike-slip tectonics in the basement of northern Nigeria. The bimodal Toro Complex is therefore considered as a late Pan-African syntectonic pluton.
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The Brazilian Coastal Mobile Belt in Southern Espirito Santo consists of Archean to Upper Proterozoic granulite-facies rocks, amphibolite-facies gneisses, and migrmtites. Towards the end of the Brasiliano Event (680-4S0 Ma) numerous calc-alkaline plutons intruded these deepseated rocks. They form eilliptic (50-150 km 2 ) bodies with cornplex, roughly concentric internal structures: basic rocks predominate in the center, whereas acidic rocks become more dominant in the outer portions. Mixing and commingling of contrasting magmas is a widespread phenomenon in these plutons. The Santa Angelica pluton, of the most complex and best investigated intrusions in this area, exhibits spectacular commingling structures: schlieren and banded textures of granite within hybrid rocks, as well as schol1en of hybrid rock within granite preserve different stages of mechanical mixing. Fine-grained hybrid rocks show phenocrysts from both gabbroic and granitic parental magmas, Phenocrysts have not been in equilibrium with the hybrid melt and therefore show corrosion, corona, and/or mantling structures. A model is presented for the Santa Angelica pluton: mantle derived basic magma of trasitional or alkali basalt affinity intruded the lower crust causing anatexis and production of granitic melts. Convection during diapiric ascent led to commingling of the contrasting magmas. Different stages of homogenization are preserved within the hybrid rocks which originated from commingling. They now represent frozen-in magma mixing.
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The Borborema Province had always been envisaged as a Brasiliano unit, and really it is it, in despite of the lack of data for the age of their supracrustal rocks. A polycyclical behavior of evolution has always been admitted for this region, based only on Rb-Sr and K-Ar data. This paper is focussing the impact on the regional geological knowledge caused by the introduction during this decade of geochronological determinations with U-Pb and Sm-Nd methods, as well as it will emphasize the coherence of the obtained results with these methods and some preexisting Rb-Sr ages (some new Rb-Sr data will be added), specially on orthogneisses. It was based on Rb-Sr data that the occurrence of an Wilson Cycle could firstly be pointed now out for this region, southwards of Patos lineament. This cycle, henceforth being postulated and named as (name of a very large physiographic part of the Northeast, where these new data are conspicuos) displays lithogenetic records from the Late Mesoproterozoic (ca. 1100 Ma, rifting processes) up to the beginnings of the Neoproterozoic (ca. 950 Ma, late collisional events). Superposition of a complex orogenic history (post - 950Ma) is recognized ali over the province, to which the name of Brasiliano Cycle is usually attributed, what is now demanding be reviewed, as well as ali their steps have to be discriminated. The identification of Cariris Velhos Cycle brings strong implications to the regional knowledge, and to the continental evolution - possibilities of correlation with Occidentalia and Sunsas-Aguapei, Mesoproterozoic terranes of the westem part of the continent. This is also true in terms of the worldwide evolution of the continental landmasses (Grenville orogeny, Rodinia, Pannotia and Gondwana supercontinents). Such facts, from now on, are opening an wide fan of questions and research lines for the Borborema Province.
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Three very large high -K calc-alkaline balholiths (Bodoco, Serra da Lagoinha, and llaporanga) intruded amphibolite faci es metamorphics of the basement adjacent to the northern boundary of the Precambrian Cachoeirinha-Salgueiro Fold Belt, west of Pernambuco and Parafba states, northeastern Brazil. These bodies were intruded syntectonically to the Fl d~formation with a final diapiric emplacement. In the ltaporanga batholith, the beSI studied of these bodies, three petrographic fades were mapped. A hybrid zone, characterized by chemical and mechanical mixing, composes most of the outer portion of the batholilh, where int~nse interaction between granite, granodiorite and diorite mdrs took place, giving rise 10 migmatitic· like features. A commingling facies wh~re granite to granodiorite and diorite units may be well individualized, is located towards the cemer of the batholith. Finally, areas of porphyritic fades are mO!~ abundant in the hybrid zone than in the commingling one. In the hybrid zon~, diorile shows 50m~tim~5 pillow like structures evidendng quenching, confirmed by the presence of fine.grained fades and adcular apatite crystals. Cuspate COntacts between diorite dikes and porphyritic granodiorites portray the viscosity contrast between these coexisting melts. Obstacles to mixing were very often overcome, judging from the el
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Research Article| June 01, 1956 STRUCTURE AND PETROLOGY OF ENCHANTED ROCK BATHOLITH, LLANO AND GILLESPIE COUNTIES, TEXAS ROBERT M HUTCHINSON ROBERT M HUTCHINSON DEPARTMENT OF GEOLOGY, KANSAS STATE COLLEGE, MANHATTAN, KANSAS. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1956) 67 (6): 763–805. https://doi.org/10.1130/0016-7606(1956)67[763:SAPOER]2.0.CO;2 Article history received: 08 Jun 1954 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT M HUTCHINSON; STRUCTURE AND PETROLOGY OF ENCHANTED ROCK BATHOLITH, LLANO AND GILLESPIE COUNTIES, TEXAS. GSA Bulletin 1956;; 67 (6): 763–805. doi: https://doi.org/10.1130/0016-7606(1956)67[763:SAPOER]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Enchanted Rock batholith is one of several moderate-sized plutons that intruded a framework of tightly folded Precambrian metamorphic rocks in Middle Precambrian time. Tectonic features within the mass and structural attitude of wall rocks indicate the northern third is phacolithic. The remainder is both discordant and concordant batholithic, although the schists show a tendency to box the compass around the batholith. The phacolithic part occupies a synclinal trough plunging 35°–40° SE.The batholith consists of four concentric zones: (1) outer zone, medium to coarse leucogranite and granite; (2) intermediate zone, medium to coarse granite and quartz monzonite; (3) intermediate central zone, coarse quartz monzonite and alkalic granodiorite; and (4) the core, fine to medium leuco-quartz monzonite and leucogranite. Rapakivi are most abundant in zone 3. Seriate porphyritic texture prevails in all zones except the core where it is increasingly hiatal. Chilled border rocks are 10–20 feet wide and consist of leuco-quartz monzonite and quartz monzonite. Apophyses of the batholith in metamorphic wall rocks are leuco-quartz monzonite and leucogranite. Hiatal porphyritic texture prevails in chilled border and apophyses rocks.Internal structures of the batholith and its relation to country-rock structures indicate intrusion during the late, most severe stages of regional compression. Tensile stresses along axial planes of folds exerted structural control and guided the rise of the magma, as indicated by the pear-shaped outline of the batholith. The most probable mechanism of magma generation is selective fusion (melting) of random rock material. This occurred at depth in down-buckled zones below the present position of the batholith.Concentration of the proportions of normative constituents of individual rock zones is strikingly grouped about a composition having a 1:1:1 ratio of albite: orthoclase: quartz. Mineralogically there are systematic gradations in proportions of plagioclase, microcline, and quartz between rock zones suggesting that unmixing of sodic plagioclase from microcline was a dominant process.Outline shape of rock zones, textural relations between plagioclase and microcline crystals, and maxima-minima concentrations of unmixed sodic plagioclase suggest migration of part of the unmixed material within the batholith during the later stages of its structural and intrusive history. Field and chemical evidence indicate that probably not more than 5 per cent of the granites of the batholith are of replacement origin.Failure of roof rocks with partial escape of volatiles, cessation of regional tectonism, and depletion of the magma reservoir in the late stages of intrusion probably are responsible for finer grain of the core. Age determinations by the "Larsen Method" give an average age for the batholith of 815 million years. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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High-K calc-alkalic plutons represent a significant proportion of the abundant magmatic bodies that intruded Borborema province (BP) of northeastern Brazil during the Neoproterozoic Brasiliano (Pan-African) orogeny. They consist of an association of mafic to intermediate (diorites to granodiorites) and felsic rocks (coarse-grained to porphyritic quartz monzonites to granites). Field and petrographic evidence indicates that the felsic and mafic rocks coexisted as contemporaneous melts, and major- and trace-element data favor magma mixing over fractional crystallization as the main petrogenetic process responsible for the petrographic and geochemical variability of these rocks. Major- and trace-element, oxygen-isotope, and radiogenic-isotope (Sr and Nd) data suggest that (1) the main source rocks of the granitoids are lower-crustal amphibolites having rare-earth-element (REE) and isotopie characteristics similar to the associated mafic rocks and (2) the source region of the diorites is the metasomatized subcontinental lithospheric mantle. These inferences imply that crustal growth occurred during the Brasiliano orogeny. Dewatering of the mantle and lower crust and addition of consolidated mafic rocks and I-type granitoids to the middle crust certainly strengthened the entire lithosphere, thus contributing to the final cratonization of the BP. Field evidence indicates that the BP high-K calc-alkalic plutons were emplaced in an intracontinental setting, implying that this magmatism was not subduction-zone related. Although the plutons are spatially associated with transcurrent shear zones, the scale of magmatism is too broad to be assigned to shear heating. 40Ar/39Ar data indicate that large areas of the BP underwent slow cooling, unlike orogenic belts where delamination or convective removal of the lithosphere occurred. Therefore, only large convective instabilities in the sublithospheric mantle may explain the thermal anomaly responsible for melting in the BP. It is proposed that a mantle plume impinging the base of the continental lithosphere under the BP may represent such a laterally extensive and long-lived heat source.
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