Leucocratic Monzogranite Research Articles

Phase equilibria constraints on crystallization differentiation: insights into the petrogenesis of the normally zoned Buddusò Pluton in north-central Sardinia

Abstract The Buddusò Pluton in NE Sardinia (Italy) is a normally zoned intrusion composed of three units with chemical composition ranging from hornblende-bearing tonalites (SiO 2 ∼ 65 wt%) to leucocratic monzogranites (SiO 2 ∼ 76 wt%). Zircon crystals in the pluton are dated at 292.2 ± 0.7 Ma and have ε Hf values ranging from −4 to −8, with no systematic differences observed between the units. The pluton, which is isotopically homogeneous at the whole-rock scale in terms of Sr and Nd isotopes, shows textural evidence indicating local crystal–melt segregation. In this paper, we have implemented a novel approach based on path-dependent phase-equilibria modelling to test the hypothesis that the internal chemical variability of the pluton was generated by crystallization differentiation of a homogeneous parental magma. Our modelling indicates that this hypothesis is valid if the mechanism by which this occurs is compaction in a rheologically locked crystal-rich magma and if the separation occurs at 0.3 GPa from a tonalitic magma with water content >2 wt%. Finally, a subset of the magmatic enclaves in the pluton are considered to be autoliths, formed by the disruption of the compacted crystal mush and interaction between these cumulates and the felsic melt.

Petrologia do Batólito Sítios Novos, Sistema Orogênico Sergipano, Província Borborema, NE do Brasil

O Batólito Sítios Novos (110 km2) é uma intrusão alongada na direção NE-SW, com idade de 631 ± 4 Ma, cuja forma concorda com a orientação regional, e se localiza na região sul do Domínio Poço Redondo, Sistema Orogênico Sergipano. Esse batólito é constituído de monzogranitos leucocráticos com textura alotriomórfica equigranular e inequigranular, e ocasionalmente por diques pegmatíticos. Enclaves máficos são raros. A região sul do batólito é afetada por zona de cisalhamento regional, e os monzogranitos adquirem estruturas milonítica e gnáissica. Oligoclásio e albita são os plagioclásios presentes nesses granitos, e a microclina ocasionalmente é pertítica. A biotita é o mineral máfico dominante e a sua composição indica afinidade com suítes orogênicas (cálcio-alcalina e peraluminosa). Zircão, titanita, magnetita, ilmenita, apatita, torita, allanita e monazita ocorrem como minerais acessórios. Bastanesita e barita ocupam frequentemente microfraturas nessas rochas. A geoquímica dos monzogranitos evidencia que são rochas fracionadas (SiO2 > 68%), peraluminosas a metaluminosas, e alocam-se em diagramas geoquímicos no campo dos granitos do Tipo I. Os espectros dos elementos terras raras apresentam fracionamento moderado (15 < LaN/YbN < 70), com anomalia negativa em Eu sugestivo de fracionamento de plagioclásio. Em diagramas multielementares, as amostras desse batólito apresentam vales pronunciados em Nb, P, Ba e Ti, e picos mais ou menos acentuados em Th, Pb e Zr. Essa assinatura geoquímica é característica de magmas orogênicos. A fonte provável para os monzonitos estudados é uma crosta de composição tonalítica.

Jardim do Seridó Suite: first example of Ediacaran peraluminous magmatism in the Rio Piranhas-Seridó Domain, Borborema Province, Northeast Brazil

Peraluminous granitoids were recently mapped in the Rio Piranhas-Serido Domain, the northeastportion of the Borborema Province. These bodies occur as stock and intrusive dikes in the metapelites of the Serido Formation, constituting the Jardim do Serido Suite. This suite has two facies. The first and most expressive facies consists of a medium-grained and whitish leucocratic monzogranite with muscovite, garnet, and biotite, while the second is represented by fine-grained and greyish leucocratic granodiorites with biotite, cropping out mainly on the edges of the main body. The thermobarometric conditions for the crystallization of the monzogranites were estimated between 2-5 kbar and 690-950 oC, whereas the lithochemical data indicate a syn-collisional peraluminous signature. Neoproterozoic garnet-biotite schist xenoliths of the Serido Formation are sometimes observed partially assimilated in the main body (stock). A U-Pb zircon age of 592 ± 2 Ma obtained in the main body is considered as the crystallization age of the Jardim do Serido Suite, corroborating the proposed existence of peraluminous magmas associated with the Brasiliano Orogenesis in the Rio Piranhas-Serido Domain. The chemical and petrographic data identified this suite as fertile to that it can generate dikes of rareelements-bearing pegmatites.

O Stock Flores: Exemplo de magmatismo granítico tipo-A no Domínio Rio Piranhas-Seridó, NE da Província Borborema

O Stock granítico Flores intrude gnaisses paleoproterozoicos, relacionados ao Complexo Caicó, no Domínio Rio Piranha-Seridó, porção setentrional da Província Borborema, NE do Brasil. Este trabalho tem como objetivo a caracterização petrológica e geoquímica de seu monzogranitos. O Stock Flores compreende duas fácies petrográficas de monzogranitos equigranulares, leucocrá- ticos: i) monzogranitos rosados de granulação media; e ii) monzogranitos cinza rosados de granulação fina. Localmente ocorrem no Fácies Tipo II, enclaves máficos exibindo feições de coexistência de magmas (mixing e mingling), e schlieren elipsoidais, interpretados como estruturas formadas por convecção magmática. Biotita é o máfico principal; titanita, allanita, apatita, zircão e opacos ocorrem como minerais acessórios; fluorita, clorita e saussurita ocorrem como minerais tardios relacionados a fluídos em estágio subsolidus. As análises geoquímicas evidenciam a natureza evoluída do Stock e afinidade alcalina com teores de SiO2 > 71,0%, e Na2 O+K2 O ~ 9,0%. Os diagramas discriminantes de ambiente tectônico demonstram a tipologia de granito Tipo-A. Tais dados são coerentes com a idade U-Pb 553±4 Ma encontrada para o Stock, o colocando dentro do contexto tardi- a pós-tectônico em relação à Orogênese Brasiliana/Pan-Africana na Província Borborema.

Crystallization conditions of porphyritic high-K calc-alkaline granitoids in the extreme northeastern Borborema Province, NE Brazil, and geodynamic implications

An integrated textural and chemical study on amphibole, biotite, plagioclase, titanite, epidote, and magnetite was conducted in order to estimate crystallization conditions, along with possible geodynamic implications, for six Ediacaran porphyritic high-K calc-alkaline granite plutons (Monte das Gameleiras, Barcelona, Acari, Caraúbas, Tourão, and Catolé do Rocha) intrusive into Archean to Paleoproterozoic rocks of the São José do Campestre (SJCD) and Rio Piranhas-Seridó (RPSD) domains, northern Borborema Province. The studied rocks include mainly porphyritic leucocratic monzogranites, as well as quartz-monzonites and granodiorites. Textures are marked by K-feldspar megacrysts (5–15 cm long) in a fine-to medium-grained matrix composed of quartz, plagioclase, amphibole, biotite, as well as titanite, epidote, FeTi oxides, allanite, apatite, and zircon as accessory minerals. Amphibole, biotite and titanite share similar compositional variations defined by increasing Al and Fe, and decreasing Mg contents from the plutons emplaced into the SJCP (Monte das Gameleiras and Barcelona) towards those in the RPSD (Acari, Caraúbas, Tourão, and Catolé do Rocha). Estimated intensive crystallization parameters reveal a weak westward range of increasing depth of emplacement, pressure and temperature in the study area. The SJCD plutons (to the east) crystallized at shallower crustal depths (14–21 km), under slightly lower pressure (3.8–5.5 kbar) and temperature (701–718 °C) intervals, and high to moderate oxygen fugacity conditions (+0.8 < ΔFQM < +2.0). On the other hand, the RPSD plutons (to the west) were emplaced at slightly deeper depths (18–23 km), under higher, yet variable pressures (4.8–6.2 kbar), temperatures (723–776 °C), and moderate to low oxygen fugacity conditions (−1.0 < ΔFQM < +1.8). These results reinforce the contrasts between the tectono-strutuctural domains of São José do Campestre and Rio Piranhas-Seridó in the northern Borborema Province.

Age and anatomy of the Gongga Shan batholith, eastern Tibetan Plateau, and its relationship to the active Xianshui-he fault

The Gongga Shan batholith of eastern Tibet, previously documented as a ca. 32–12.8 Ma granite pluton, shows some of the youngest U-Pb granite crystallization ages recorded from the Tibetan Plateau, with major implications for the tectonothermal history of the region. Field observations indicate that the batholith is composite; some localities show at least seven crosscutting phases of granitoids that range in composition from diorite to leucocratic monzogranite. In this study we present U-Pb ages of zircon and allanite dated by laser ablation–inductively coupled plasma–mass spectrometry on seven samples, to further investigate the chronology of the batholith. The age data constrain two striking tectonic-plutonic events: a complex Triassic–Jurassic (ca. 215–159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14–5 Ma) record of monzonite-leucogranite emplacement. The former age range is attributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduction zone magmatism along the western Yangtze block of south China. The younger component may be related to localized partial melting (muscovite dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, and are among the youngest crustal melt granites exposed on the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of zircons and growth and/or resetting of allanites. The young garnet, muscovite, and biotite leucogranites occur mainly in the central part of the batholith and adjacent to the eastern margin of the batholith at Kangding, where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seismically active strike-slip fault in Tibet and is thought to record the eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites of the Gongga Shan massif and has a major transpressional component in the Kangding-Moxi region. The course of the Xianshui Jiang river is offset by ∼62 km along the Xianshui-he fault and in the Kangding area granites as young as ca. 5 Ma are cut by the fault. Our new geochronological data show that only a part of the Gongga Shan granite batholith is composed of young (Miocene) melt, and we surmise that as most of eastern Tibet is composed of Precambrian–Triassic Indosinian rocks, there is no geological evidence to support regional Cenozoic internal thickening or metamorphism and no evidence for eastward-directed lower crustal flow away from Tibet. We suggest that underthrusting of Indian lower crust north as far as the Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau.

Petrology and Geochemistry of the Late Eocene Harrison Pass Pluton, Ruby Mountains Core Complex, Northeastern Nevada

which suggest a single mixing event before emplacement in the upper The late Eocene Harrison Pass pluton was emplaced in the crust. Late-stage activity consisted of three pulses of monzogranitic transition zone between the infrastructure and suprastructure of the magma plus sparse mafic dikes. The largest and youngest of these Ruby Mountains core complex. Emplacement was at >3 kbar pulses, the two-mica monzogranite of Green Mountain Creek, is pressure and was in two stages: early stage tonalitic to monzogranitic distinct from all other units in the presence of restitic enclaves, low magmas, followed by late-stage monzogranites and mafic dikes. Nd, and high initial Sr/Sr. Pod-like bodies of leucocratic biotite The early stage began with emplacement of biotite ± hornblende ± amphibole monzogranite and sheets and dikes of leucocratic granodiorite of Toyn Creek, followed by the biotite monzogranite of two-mica monzogranite make up the other late-stage granites. These Corral Creek. Quenched equivalents of these units are preserved as rocks display deep negative Eu anomalies and show wide, nonporphyritic dikes in the roof. Leucocratic monzogranite that forms systematic concentrations of high field strength elements; however, cupolas in the roof zone represents the product of fractional crystheir isotopic compositions are identical to those of the early stage tallization of the early magmas. Al-in-hornblende barometry and rocks. They are thought to be small melt fractions of the lower to the presence of magmatic epidote suggest that early stage magmas middle crust. Their elemental compositions are thought to result resided at a pressure of >5−6 kbar before emplacement in the from the effects of residual plagioclase and accessory minerals. The upper crust. Compositional variation in the Toyn Creek and Corral variable and non-systematic isotopic compositions of all granitic Creek units is essentially linear, and can be explained by mixing units in the pluton suggest a heterogeneous source region, such as of a tonalitic end member with a monzogranitic end member such the Proterozoic Mojave province. as evolved samples of the monzogranite of Corral Creek. The tonalitic end member was itself a hybrid that formed by interaction

The Variscan calc-alkalic plutonism of western Corsica: mineralogy and major and trace element geochemistry

Petrographic and structural observations on the calc-alkalic plutonism of western Corsica revealed the existence of several successively emplaced units associated with large basic bodies. The present mineralogical and geochemical study deals with the genesis, evolution and relationships of these different units. Basic plutonism is represented by three genetically linked types of rock: norites and troctolites with cumulate textures characterized by low REE contents and either no Eu anomaly or a positive Eu anomaly; gabbros with enriched LREE relatively to HREE patterns, probably close to an initial basaltic liquid; and diorites ranging up to charnockites which represent liquids evolved to varying degrees, mainly by fractional crystallization. Trace element data and studies on the evolution of pyroxene pairs demonstrate the consanguinity of these calc-alkaline basic rocks which are derived from a high alumina basaltic melt. The various granitoids (granodiorites, monzogranites and leucocratic monzogranites, i.e., adamellites) have distinct evolution trends as shown by the composition of their mafic minerals and by trace element distributions. They cannot be considered as being derivatives of the basic suite and they cannot be related by a common fractionation sequence. Rather, they represent distinctive batches of crustal anatexis. In addition, hybridization phenomena with the basic melt are noticed in granodiorites. The particular problem of the low La/Yb, Eu/Eu∗ and the high U, Th, Cs leucocratic monzogranites is discussed in detail. In addition to more conventional trace element diagrams, the simultaneous statistical treatment of all the geochemical data by correspondence factor analysis is shown to be a very use tool in distinguishing between the different units and to classify the elements according to their geochemical properties.

General geology and petrography of the precambrian basement around Zalingei town, Darfur Province, Sudan

Abstract The exposed Precambrian rocks of the greater Zalingei area consisted originally of a thick succession of psammites and semipelites. The upper exposed part of this succession included some probable bituminous intercalations that are now represented by graphitic quartz schists. The succession also contained few carbonate beds and minor intrusions of dolerite. The entire succession had later suffered a major protracted cycle of tectonic deformation, regional metamorphism and migmatism. The metasediments were open-folded into synforms and antiforms that now have ENE-WSW axial trends. The migmatites around Zalingei town display repeated events of deformation and recrystallization, as well as partial melting. Earlier metamorphic assemblages point to granulite facies conditions (sillimanite, orthorhombic pyroxene) but these assemblages were largely replaced by later amphibolite facies minerals (kyanite, staurolite, almandite, hornblende). Partial melting accompanied by extensive flow-folding on all scales affected the more susceptible semipelites and produced a wide variety of migmatitic gneisses. At first these migmatites were oligoclasic and produced melt segregates and small syntectonic injections of trondhjemite, tonalite or granodiorite. Later, more potassic migmatites evolved, mainly through microclinization of the earlier migmatites. A monzogranitic intrusion with very large porphyritic potash feldspars was also mobilized from a lower crustal level. A further episode of some strong deformation followed and left a variable foliate and rather erratic imprint on the different rocks. This deformation might have accompanied the uprise of intrusive granitoid diapirs mantled from outside by diverse concentrically foliated gneisses with boudinaged minor pegmatite intrusions. The diapirs have inner massive central cores of leucocratic monzogranites and their intrusion ended the major plutonic activities. Quartz—tourmaline veinlets and massive dyke-like bodies accompanied and survived the diapir intrusions and represented the terminal penumatolytic and hydrothermal stages. The large quartz dykes, the master-joints and other late lineaments are crudely oriented in a NW-SE trend. From apparently similar lithologies and geologic events in the adjacent basement rocks of Eastern Chad and the Central African Republic, the original age of the Zalingei sedimentary succession is provisionally assigned to Precambrian D (older than 2500 Ma). The migmatitic to late diapiric activities, however, represented reactivation (rejuvenation) of the basement rocks as the Zalingei basement falls within the broad NE-SW mobilized belt running between the West African and Congo cratons in Pan-African times (500–600 Ma).