Metaluminous Granitoids Research Articles

Permian, rifting related fayalite syenite in the Panxi region, SW China

Within the Panxi rift of the Permian Emeishan large igneous province (ELIP), SW China, a 20 km 2 quartz-bearing fayalite syenitic pluton intrudes the 260 Ma Baima igneous complex (BIC). The fayalite syenite is dated using SHRIMP zircon U–Pb techniques at 252 ± 2.5 Ma. The Panxi fayalite syenite is mainly composed of alkali feldspar, ferroan augite, fayalite, quartz and Fe–Ti oxides and contains 61.8–63.7 wt.% SiO 2, 5.0–7.3 wt.% Fe 2O 3 t and 9.9–12.1 wt.% total alkalis, belonging to an A-type variety of ferroan alkalic metaluminous granitoids. The fayalite syenite displays positive Eu anomalies (Eu/Eu ⁎ = 0.9–8.9) and is enriched in LREE [(La/Yb) N =6.6–11.5]. The ɛNd values and ( 87Sr/ 86Sr) initial ratios of the fayalite syenite range from + 1.3 to + 1.9 and from 0.7035 to 0.7068, respectively. The fayalite syenite and the BIC had the same mantle source; however the 8 myr age gap between them suggests the fayalite syenite formed by remelting of underplated mafic magmas injected into the crust during ELIP magmatism. The ages of the fayalite syenite and BIC are coincident with two Permian mass extinction events and suggest that ELIP may have influenced both.

Petrology and geochemistry of paleoproterozoic A-type granite at Kanigiri in the Nellore–Khammam schist belt, Andhra Pradesh, India

The Kanigiri Pluton (6.2 × 2.1 km 2; 995 Ma) occurs close to a major fault and is intrusive into the meta-volcanics in the western margin of the Nellore–Khammam schist belt in Andhra Pradesh, India. The dominantly metaluminous pluton displays geochemical characteristics of A-type granites, including high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu, displaying distinct variation trends with increasing silica content. It has less fractionated REE patterns and large negative Eu anomalies. The cluster analyses of major and trace elements indicate three main associations: (a) A primary magmatic association characterized by the presence of La, Ce, Fe, Ti, Mn, K, Ba, and Sr, (b) Ore related association consisting of Zr, Hf, Nb, Y, and (c) a volatile association with Rb, Ta, Li, and Cs. Systematic variation in Sr, Rb/Sr, Rb/Ba ratios along with and low K/Rb ratios, reflects fractionation of feldspars. Variations in Sc, and Ta result from fractionation of ferromagnesian silicates. Removal of zircon and allanite affected the Hf/Ta and Ce/Yb ratios. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with the results of melting experiments and indicates that metaluminous granitoids of this pluton were probably derived through melting of lower crustal sources. Geochemical, field and petrological data for the Kanigiri granite suggest that the granite is a partial melting product of mafic rocks like gabbros that are also present in the study region.

Ridge Subduction-Related Jurassic Plutonism in and around the Okcheon Metamorphic Belt, South Korea, and Implications for Northeast Asian Tectonics

The Okcheon metamorphic belt (OMB) in central South Korea is surrounded by Middle Jurassic granitoid batholiths that intruded South Korea extensively; the granitic bodies form a complex about 200 km long and 150 km wide as part of a Mesozoic granite belt along the East Asian continental margin. Middle Jurassic magmatism was related to ridge subduction that occurred around 200 to 166 Ma, with the main magmatic period between 175 and 166 Ma; main cooling ages range from 168 to 152 Ma. The magmatism was divided into two stages: (1) a deeper, earlier stage, which resulted in emplacement of diorite, granodiorite and granite as shown in the northeast OMB; and (2) a shallower, younger stage, which resulted in emplacement of granite and two-mica granite as shown in the southwest OMB. Most granitoids are peraluminous to metaluminous I-type granitoids that originated in a volcanic arc; an exception is an S-type two-mica granite. Inherited cores of 998 and 262 Ma U-Pb SHRIMP II zircon ages from the two-mica granite indicate that two-mica granite is reworked crustal material formed by earlier magmatism before the Middle Jurassic event. Together with previous studies on the Middle Jurassic granitoids, the present result indicates that subduction of the Farallon-Izanagi ridge beneath Asia caused widespread igneous activity throughout South Korea, especially during Middle Jurassic ridge subduction.

Transition from arc- to post-collision extensional setting revealed by K-Ar dating and petrology: an example from the granitoids of the Eastern Pontide Igneous Terrane, Araklı-Trabzon, NE Turkey

The Eastern Pontide Igneous Terrane (EPIT) includes several Cretaceous to Neogene intrusive rocks, ranging in composition from low-K tholeiitic gabbros through calc-alkaline and high-K calc-alkaline metaluminous granitoids or peraluminous leucogranites to alkaline syenites. Such high diversity in age and composition is also accompanied by a broad spectrum in terms of geodynamics—i.e. from arc through syn-collisional thickening to post-collisional extensional regimes. Shallow-seated porphyritic acidic to intermediate rocks are from oldest to youngest, on the basis of field relations, the Gundogdu altered microgranite, the Bogali K-feldspar-megacrystic monzogranite and the Uzuntarla porphyritic granodiorite. These rocks, exposed in the southern part of the Arakli region, east of Trabzon, Turkey, were studied in terms of their mineralogy and petrography, whole-rock geochemistry and hornblende K–Ar dating. The mineralogical and geochemical data reveal an apparent diversity in incompatible-element enrichment and depletion, for the Bogali unit and Uzuntarla unit, respectively. The Bogali and Uzuntarla units yield hornblende K–Ar ages ranging from 75.7 ± 1.55 to 61.4 ± 1.47 Ma and from 42.4 ± 0.87 to 41.2 ± 0.89 Ma, respectively. The diversity in both mineralogy–geochemistry and hornblende K–Ar ages suggests that the Bogali and Uzuntarla units are parts of the Cretaceous arc and Eocene post-collision extensional-related igneous activity, respectively, in the EPIT of northern Turkey. Copyright © 2005 John Wiley & Sons, Ltd.

Variety and genesis of the pyroxene-bearing S- and I-type granitoids from the Hidaka Metamorphic Belt, Hokkaido, northern Japan

ABSTRACTThe high-dT/dP-type Hidaka Metamorphic Belt in Hokkaido, northern Japan, represents a tilted crustal section of a magmatic arc of Tertiary age. The highest metamorphic grades reached are granulite facies, and the syn-metamorphic granitic rocks are widely distributed in this metamorphic terrane. The granitic rocks are mainly tonalitic and granodioritic in composition, and are classified into peraluminous (S-type) and metaluminous (I-type) granitoids. A large amount of pyroxene-bearing S-type tonalites (garnet-orthopyroxene tonalite) is distributed in the Niikappu river region in the northern part of the Hidaka Metamorphic Belt. Pyroxene-bearing I-type tonalite (two-pyroxene hornblende tonalite) bodies are also distributed in this area.The pyroxene-bearing tonalites are classified into several sub-types on the basis of their field occurrence, texture, mineral assemblage and geochemical features. Homogeneous IH- and SH-type tonalite are thought to represent original magmas, i.e. those which have been generated by partial melting of mafic metamorphic rocks and pelitic-psammitic metamorphic rocks, respectively. Model calculations assuming batch partial melting indicate that possible restites are garnet-two-pyroxene mafic granulite for IH-type and garnet-orthopyroxene aluminous granulite for SH-type. The unexposed lowermost crust of the ‘Hidaka crust’ is thought to be composed of garnet-two-pyroxene mafic granulite, garnet-orthopyroxene aluminous granulite and metagabbros.

Neoproterozoic A-type granitoids of the central and southern Appalachians: intraplate magmatism associated with episodic rifting of the Rodinian supercontinent

Emplacement of compositionally distinctive granitic plutons accompanied two pulses (765–680 and 620–550 Ma) of crustal extension that affected the Rodinian craton at the present location of the central Appalachians during the Neoproterozoic. The dominantly metaluminous plutons display mineralogical and geochemical characteristics of A-type granites including high FeOt/MgO ratios, high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu. These dike-like, sheet complexes occur throughout the Blue Ridge province of Virginia and North Carolina, and were emplaced at shallow levels in continental crust during active extension, forming locally multiple-intrusive plutons elongated perpendicular to the axis of extension. New U–Pb zircon ages obtained from the Polly Wright Cove (706±4 Ma) and Suck Mountain (680±4 Ma) plutons indicate that metaluminous magmas continued to be replenished near the end of the first pulse of rifting. The Suck Mountain body is presently the youngest known igneous body associated with earlier rifting. U–Pb zircon ages for the Pound Ridge Granite Gneiss (562±5 Ma) and Yonkers Gneiss (563±2 Ma) in the Manhattan prong of southeastern New York constitute the first evidence of plutonic felsic activity associated with the later period of rifting in the U.S. Appalachians, and suggest that similar melt-generation processes were operative during both intervals of crustal extension. Fractionation processes involving primary minerals were responsible for much of the compositional variation within individual plutons. Compositions of mapped lithologic units in a subset of plutons studied in detail define overlapping data arrays, indicating that, throughout the province, similar petrologic processes operated locally on magmas that became successively more chemically evolved. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with results of melting experiments and indicates that metaluminous granitoids of the supersuite likely were derived through melting of lower crustal sources. Mildly peralkaline rocks of the Robertson River batholith and Irish Creek pluton may be derived from more chemically primitive sources similar in composition to ocean–island basalts. Blue Ridge granitoids define a plutonic episode that occurred during an unsuccessful pulse of crustal extension which predated opening of Iapetus by more than 100 million years. Granitoid gneisses in New York were emplaced during an extension-related, dominantly mafic magmatic episode that ultimately led to development of Iapetus.

Oxygen isotope compositions and magmatic epidote from two contrasting metaluminous granitoids, NE Brazil

Oxygen isotope compositions of mineral separates were determined from two metaluminous granitoids (Emas and Sao Rafael plutons) from northeastern Brazil. The I-type Emas pluton has high δ18O (WR) values (11.5–11.8‰), whereas the Sao Rafael pluton has low δ18O (WR) values (7.5–8.1‰), but Sr and Nd are characteristics of S-type granitoids. Measured mineral–mineral fractionations suggest continuous sub-solidus inter-mineral isotope exchange among all minerals except zircon. There is a large and consistent quartz–epidote fractionation that gives apparent temperatures that are much lower than anticipated closure temperatures for epidote. Oxygen isotope fractionation between natural zircon and magmatic epidote is opposite to that predicted from theoretical determinations, as δ18O (epidote) <δ18O (zircon). An empirical calibration based on these results would suggests a closure T for oxygen in epidote of ~500 °C and Δ(qtz–epi) ~5.19 at 500 °C.

Mafic microgranular enclaves: early segregation from metaluminous magma (Sierra de Chepes), Pampean Ranges, NW Argentina

Mafic microgranular enclaves (MME) were formed in metaluminous granitoids from Sierra de Chepes, Pampean Ranges, NW Argentina. The granitoids were emplaced in low-pressure conditions during the Lower Ordovician. The MME seem to have played an important role in the evolution of metaluminous magmas, as has been emphasized in several petrological works. Fieldwork, combined with petrographical, geochemical, and microprobe data, leads to the conclusion that the MME result from early crystallization (mainly hydrous mafic minerals, plagioclase, and some accessory minerals). Early segregation of the MME led to differentiation of the parental granitic magma by fractional crystallization. The MME studied here have isotopic compositions remarkably similar to those of their host granitoids, which suggests that they represent cogenetic suites derived from the same lithospheric source, with crustal residence ages ∼1700 Ma. In contrast, the MME lodged in Andean granitoids during the Mesozoic–Tertiary (e.g. Papudo–Quintero complex, Chile) represent cogenetic suites of primitive mantle origin. Thus, the characteristics of the Ordovician granitoids could help define the geotectonic settings in which MME represent early products of crystallization without significant asthenospheric mantle participation.

Low-pressure emplacement of epidote-bearing metaluminous granitoids in the Sierra de Chepes (Famatinian Orogen, Argentina) and relationships with the magma source

A petrological and geochemical study of Lower Ordovician epidote-bearing granitoids of the Sierra de Chepes (Famatinian Orogen, Sierras Pampeanas, Argentina) indicates low-pressure emplacement of these extensive metaluminous suites. Textural relationships combined with major oxide and REE data support a magmatic origin for the ubiquitous epidote in hornblende-biotite granodiorite and tonalite plutons. Geothermobarometry using mineral assemblages in igneous rocks indicates a temperature interval from 780 to 850°C, and low emplacement pressures in the range of 2.2 to 4.1 kbar, consistent with those estimated from the metasedimentary envelope of the plutons. The widespread occurrence of primary epidote is a distinctive characteristic of the Lower Ordovician granitoids as opposed to the Andean Carboniferous granitoids emplaced at middle pressures ( e.g. , Santo Domingo Complex, 7 kbar), where magmatic epidote-granitoids are scarce and secondary epidote is commonly present. On the other hand, the pistacite value (Ps=[Fe 3+ /(Fe 3+ +Al)] x 100, in %) in magmatic epidote of the Ordovician granitoids (average Ps=28) sharply contrasts with that of magmatic epidote in the Carboniferous Andean granitoids (average Ps=24). This strongly suggests that the occurrence of magmatic epidote-bearing granitoids might be related to different sources, the Famatinian Orogen granitoids being mainly derived by melting of old continental lithosphere with probable partial contribution from subcontinental lithospheric mantle, and the Carboniferous Andean granitoids mainly resulting from mixing of crustal- and mantle- derived magmas. If the relationship of epidote-bearing granitoids to the characteristics of the source is confirmed in future studies, this will constrain the geotectonic environment in which the epidote-bearing magmas occur.

REE fractionation by accessory minerals in epidote-bearing metaluminous granitoids from the Sierras Pampeanas, Argentina

AbstractA study of the distribution of REE in epidote-bearing metaluminous granitoids from Sierra de Chepes, Sierras Pampeanas, Argentina, reveals that a large proportion of the REE reside in the accessory minerals (allanite, epidote, titanite, apatite and zircon), and therefore these minerals control the behaviour of REE in granitic magmas. Well-developed chemical zonation in titanite indicates that the REE content decreases in the melt during crystallization of this mineral. The textural and chemical characteristics of euhedral epidote suggest a magmatic origin, and in that case it may have played an important role in the fractionation of the REE. The amount of silica and any other geochemical parameter indicative of fractionation progress in the dominant granodioritic-tonalitic facies (gtf) do not correlate with observed variations in the REE patterns. When many accessory minerals are involved, as in the gtf, the differentiated melts (e.g. aplites) are REE poor. Thus, the presence/absence of accessory minerals in granitoids can be indicative of the generation of differentiated melt enriched or poor in REE and other trace elements. This may have an economic significance, as it may allow us to predict the probable geochemistry of the differentiated melts (i.e. those that tend to develop mineralization) from the textural analysis of the ‘regional’ granitic rock.Finally, the type and abundance of accessory minerals in the granitic suite can also help us to define the geotectonic environment where magmas were generated.

The origin and geodynamic significance of the Alpine cordierite-bearing granitoids of northern Algeria. A combined petrological, mineralogical, geochemical and isotopic (O, H, Sr, Nd) study

In northern Algeria, the coastal area displays a very variegated magmatism of Miocene age, extending E–W over 900 km with a width of less than 50 km. This magmatism includes both plutonic and volcanic massifs containing basic, intermediate to acidic rock types. Two groups may be identified on the basis of the chemistry and mineralogy. The first group has a calc-alkaline metaluminous chemistry and contains mainly “ high-K” types, sometimes “ medium-K” and exceptionally “low-K” types. They are gabbros and basalts, diorites and andesites, tonalites and dacites. Some granitoids (quartz-monzonites, granodiorites and syenogranites) possess affinities with this magmatism (Bejaia–Amizour and Thenia intrusions). Only mafic to intermediate enclaves of magmatic origin are found in granitoids from this group. The second group has strongly peraluminous compositions: they are cordierite-bearing granodiorites, monzogranites and microgranites, as well as dacites and rhyolites (ignimbrites). Two types of enclaves exist in these granitoids: (i) microgranular metaluminous hornblende-bearing enclaves of igneous derivation, (ii) strongly peraluminous enclaves of metapelitic compositions. Textures and mineralogical associations indicate that the enclaves experienced heating at decreasing pressures and also partial melting. Therefore, they are indicative of assimilation, not of restite unmixing. After removing the local effects of hydrothermal alteration, the O, Sr, Nd isotopic compositions of parent magmas were determined: they span a very large range of values, e.g., εNd (T) varies from +12 (Cap Bougaroun gabbros) to −10 (Crd-bearing granitoids). From the covariation of Sr–Nd isotopic compositions and the constraints of O isotopic ratios, the following sources are proposed for the different rock types: Mafic to intermediate rocks contain a subduction-related component but most of them require contamination at crustal levels. The source of metaluminous granitoids is predominantly crustal as constrained by their high O isotopic composition ( δ 18O ranging from +9.3‰ up to +13‰), but they might contain a minor component derived from the mafic sources. Most of the cordierite-bearing granitoids are thought to be formed through assimilation of pelitic metasediments by a less aluminous parental magma. The general tectonic setting of this Neogene (limited, linear but variegated) magmatism is explanable in terms of the slab detachment models recently advocated by several authors in the western Mediterranean domains. This study argues that cordierite-bearing granitoids may be produced in areas of high heat flow, in crustal domains which did not experience important thickening.

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post‐metamorphic cooling in the Hercynian crustal cross‐section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma−1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.

The genesis and characterisation of ‘Marmor Misium’ from Kozak (Turkey), a granite used in antiquity

‘Marmor Misium’ was one of the most used granites of antiquity, especially for the manufacture of columns and slabs. It was quarried in the area of Kozak (Turkey) and used locally from the Late Hellenistic period and later, probably at the end of the 1st century AD, exported to Rome and other important Roman towns of the Mediterranean sea. It seems that ‘Marmor Misium’ continued to be quarried until the Early Byzantine time; it is now still quarried for local use (road paving, etc.). In spite of its historical importance and geological interest, this granite has never been the subject of detailed petrological studies. That is why in this work it has been sampled from various outcrops of the plutonite and in abandoned ancient quarries and characterised petrographically and geochemically. From the petrographic point of view, the plutonites of Kozak are medium-grained biotite-hornblende granite and granodiorite composed of 34.2–53 % (modal) zoned plagioclase, 23.7–35.4 % K-feldspar, 21.4–29 % anhedral quartz, 4.9–10 % brown biotite and 7.7 % green hornblende. These rocks are I-type metaluminous granitoids of high-k calc-alkaline orogenic series forming a crystallised body at a depth of about 10 km. The positive distinction between ‘Marmor Misium’ and two other granites used in antiquity and macroscopically very similar to it, those from the Elba and Giglio islands (Italy), is easily made by considering the absence of hornblende, the peraluminous character and the lower Sr and Ba and higher Rb contents for the latter.

Marie Byrd Land, West Antarctica: Evolution of Gondwana's Pacific margin constrained by zircon U-Pb geochronology and feldspar common-Pb isotopic compositions

The Paleozoic and Mesozoic development and subsequent fragmentation of Gondwana's Pacific margin are recorded in igneous and metamorphic rocks that crop out in Marie Byrd Land, West Antarctica, recognized on geologic and paleomagnetic grounds to compose a discrete crustal block. Widespread metaluminous granitoids dated by the zircon U-Pb method as middle to late Paleozoic show that convergence-related magmatism dominated the early evolution of this margin. Dates for granodiorites, monzogranites, and granites from the Ruppert and Hobbs coasts of western Marie Byrd Land reveal a prolonged period of subduction-related calc-alkaline magmatism between at least 320 ± 3 Ma (age of the oldest granodiorite dated) and 110 ± 1 Ma (the age of the youngest I-type granitoid in the area). The latter, known as the Mount Prince granite, is intruded by swarms of mafic and intermediate dikes believed to record the onset of rifting that led to separation of the New Zealand microcontinent. The dikes have been dated by zircon U-Pb as 101 ± 1 Ma. Thus, the regime along the Ruppert and Hobbs coasts had shifted from subduction-related to rift-related magmatism within an ∼9 m.y. period. In the Kohler Range and the Pine Island Bay areas of eastern Marie Byrd Land, the calc-alkaline magmatism did not terminate until 96 ± 1 Ma, based on U-Pb dating of zircons from one granitoid sample, or 94 ± 3 Ma based on zircons from another. This evidence requires that subduction shut off from west to east, as suggested previously on the basis of geophysical models. No continental separation occurred to the east of Marie Byrd Land. The margins of the Thurston Island and Antarctic Peninsula crustal blocks went directly from convergent to inactive, except at the northernmost tip of the peninsula, where the South Shetlands Island block is actively separating. With their zircon U-Pb ages clustering around 100 ± 2 Ma, dike-free anorogenic syenites and quartz syenites along the Ruppert and Hobbs coasts show that the transition to extensional magmatism was rapid in the west. This is also reflected by the fact that from the onset of rifting at 101 ± 1 Ma to formation of oceanic crust between Marie Byrd Land and greater New Zealand (Campbell Plateau, Chatham Rise, North Island, and South Island) prior to chron 33o ca. 81 Ma required only 20 m.y. For comparison, this is only two-thirds of the ∼30 m.y. it took for the Central Atlantic to open after initial rift-related magmatism. The swiftness of the separation between Marie Byrd Land and greater New Zealand demonstrated by our data is consistent with ridge-trench interaction rather than a mantle plume as the primary cause of the breakup, as is the west to east diachroneity in the cessation of subduction. Exposures of host rocks to the erosion-resistant plutons are scarce in mostly snow- and ice-covered Marie Byrd Land. The occurrence in the zircons of widely separated granitoids of discordant U-Pb patterns we attribute to inheritance (the best-constrained upper concordia intercepts are as high as 1576 ± 55 Ma). This suggests either that stretched Precambrian basement underlies most of Marie Byrd Land, or that clastic sedimentary sequences with Precambrian detrital zircons underlie much of the margin.

Post-collisional potassic granitoids from the southern and northwestern parts of the Late Neoproterozoic East African Orogen: a review

Potassic metaluminous granitoids with enrichments of HFS elements constitute part of widespread post-collisional magmatism related to the Late Neoproterozoic Pan-African orogeny in northeastern Africa (Sudan, Ethiopia, Somalia) and Madagascar. The plutons were emplaced between 580 and 470 Ma and comprise both subsolvus and hypersolvus biotite–granite, biotite–hornblende–granite, quartz–monzonite and quartz–syenite. Pyroxene-bearing granitoids are subordinate. Basic dikes and enclaves of monzodioritic composition are locally associated with the granitoid plutons. Granitoids emplaced in pre-Neoproterozoic crust have Sr i -ratios between 0.7060 and 0.7236 and ε Nd( t) values between −15.8 and −5.6 while those emplaced in, or close to the contact with, juvenile Neoproterozoic crust have lower Sr i -ratios (0.7036–0.7075) and positive ε Nd( t) values (4.6). However, it is unlikely that the potassic granitoids represent products of crustal melting alone. The association with basic magmas derived from subduction-modified enriched mantle sources strongly suggests that the granitoids represent hybrid magmas produced by interaction and mixing of mantle and crust derived melts in the lower crust. The most intense period of this potassic granitoid magmatism occurred between 585 and 540 Ma, largely coeval with HT granulite facies metamorphism in Madagascar and with amphibolite facies retrogression in northeastern Africa (Somalia, Sudan). Granitoid magmatism and high-grade metamorphism are probably both related to post-collisional lithospheric thinning, magmatic underplating and crustal relaxation. However, the emplacement of potassic granites continued until about 470 Ma and implies several magmatic pulses associated with different phases of crustal uplift and cooling. The potassic metaluminous granites are temporally and spatially associated with post-collisional high-K calc-alkaline granites with which they share many petrographical, geochemical and isotopical similarities, except the incompatible element enrichments. The resemblance indicates a strongly related petrogenesis of both granite associations.

Fractionation of the rare earths in a suite of highly evolved metaluminous granitoids and felsic dykes from the Western Cape Province, South Africa

The rare earth element (REE) distribution in a comagmatic, metaluminous, igneous suite, consisting of a monzogranite, differentiating to an even-grained granite and eventually a sequence of dyke rocks, was investigated. During the early stages of the differentiation process, the REE behaved compatibly, the LREE being preferentially removed by fractionation of early allanite and monazite. With increasing fractionation, higher silica content and increasing alkalinity in the dyke rocks, evidence for the incompatible behaviour of the REE were obtained. LREE, Eu and, to a lesser extent, Gd were fractionated from the HREE and concentrated in the dyke centres. Partitioning of F and F-REE complexes towards the highly fractionated melt phase contributed towards this process. An increase in oxidation conditions during the very late evolutionary phases of the dykes again stabilized allanite as an accessory phase. LREE were sufficiently enriched at this stage to be incorporated in the mineral, leading to a subsequent lowering of the La Sm ratio in some highly evolved dyke centres. Late crystallizing fluorite in dyke centres accommodated some of the enriched LREE by a process of coupled substitution. Evidence for these fractionation processes in evolved metaluminous felsic rocks have implications for the use of REE in modelling the generation of such rocks.

Genesis of porphyry and plutonic mineralisation systems in metaluminous granitoids of the Grampian Terrane, Scotland

AbstractMineralisation associated with Late Caledonian metaluminous granitoids in the Grampian Terrane has been investigated using stable isotope, fluid inclusion and mineralogical techniques.A porphyry-stock-related style of mineralisation in the Grampian Terrane is characterised by a stockwork of veinlets and disseminations in dacite prophyries, consisting of quartz, dolomite, sulphides and late calcite, and well-developed wallrock alteration dominated by zones of phyllic, sericitic and propylitic alteration. On the basis of δ34S (+0·4±l·0‰), δ13C (−5·7‰ to + l·4‰) and δ18O (+10·8‰ to +19·9‰) it is likely that initial mineralising components were orthomagmatic with an input of external fluids during the later parageneses. Fluids were saline, boiling (up to 560°C), deficient in CO2, and ore deposition took place at depths of less than 3 km.Plutonic-hosted mineralisation in appinites, diorites, tonalites and monzogranites is commonly represented by sporadic disseminations and occasional veins consisting of quartz, calcite and sulphides. Wallrock alteration is generally propylitic with phyllic vein selvages. Deposition from a cooling magma sourced fluid is indicated by δ34S (+2·6±l·5‰), δ13C (−7·2‰ to −4·5‰) and δ18O (+9·5‰ to + ll·8‰) data. Fluids were CO2-rich and of low salinity; inclusions were trapped below ≈460°C, and formed at estimated depths of 3–5 km.Differences between these styles of mineralisation may due to multiple factors, the most important being the nature of the fluid: porphyry systems are dominated by greater volumes and much higher temperatures of hydrothermal fluids. Other controlling factors are likely to be the compositional characteristics of the melt source region, the mechanism of magma ascent, the level of emplacement, and the nature of the host metasediments. Variations in δ34S between the two groups are related, for the most part, to redox processes during magma and fluid genesis and not by crustal contamination.Nolarge porphyry-related mineral deposits have been found in the Grampian Terrane, unlike those in Mesozoic and Tertiary continental margin environments. This is largely due to a combination of detrimental factors which massively reducesthe probability of economic mineralisation. These include the already metamorphosed nature of the host Dalradian, the absence of seawater (which entered many subduction-related magmatic systems), a poorly-developed system of deep faults (most deposits too deep to be influenced by surface-derived fluids), and the absence of supergene enrichment. The main processes which aid the concentration of mineralisation involve encroachment of external fluids (formation, meteoric and seawaters) into the magmatic system, but these fluids were largely absent from the Grampian host block at the time of granitoid intrusion.The results of this study can be used in characterising the sources of fluids in sedimentary-hosted ore veins known (or considered) to be underlain by metaluminous granitoid batholiths, particularly in estimating the degree of magmatic fluid inputs into the vein systems: an example where this interaction has occurred (the Tyndrum Fault Zone) is discussed.

Microgranular enclaves in the Pan-African I-type granites from the Sinai Massif: petrology, mineralogy and geochemistry

A geological, petrological and geochemical study involving mineral chemistry and whole-rock major and trace element data has been carried out on the microgranular enclaves hosted in the Sinai Massif I-type granites. Enclave specimens were collected from two separated plutons along the western margin of the Sinai Massif: the northern pluton (NP) and southern pluton (SP). Enclaves consist of monzogabbros, monzodiorites and diorites, as well as, rare monzonites, syenogabbros and syenodiorites. They are characterized by wide compositional range (48–62% SiO 2). Mineral chemistry and whole-rock chemistries both suggest that enclaves from SP and NP were derived from two to three magmas. Enclaves provide microstructural evidence pertaining to magmatic source material. The origin of present microgranular enclaves hosted in the I-type metaluminous granitoids is best explained by mixing/mingling of hot basic magmas with relatively cooler acidic magma in a plutonic environment. Substantial changes in rheological properties of magmas occur due to thermal diffusion at the interfaces of these globules and increase the undercooling rate. Magma mingling occurs instead of through mixing and homogenization. The mafic globules are dispersed randomly or concentrated into swarms by flow of the hotter magma.

Kinetically induced compositional zoning in titanite: implications for accessory-phase/melt partitioning of trace elements

Usually it is assumed that the partitioning of trace elements into titanite in metaluminous granitoid plutonic environments takes place under equilibrium conditions and that compositional zoning is due solely to progressive changes in melt chemistry and/or mineral/melt partition coefficients. Examination of titanites from a variety of Caledonian metaluminous granitoids and related rocks has revealed that sector zoning is present, indicating disequilibrium partitioning. The sector zoning in titanites is defined principally by the distribution of the rare earth elements (REE), Y, Nb, Al and Fe. The REE, Y and Nb preferentially occur within the minor (100) sectors relative to the morphologically important (111) sectors. The reverse is true of Al and Fe which preferentially occur within the (111) sectors relative to the (100) sectors. The patterns of sector zoning are complicated by the fact that the relative growth rates of the various crystal faces fluctuated during growth. Sector zoning indicates that crystal-interface kinetics are responsible for the observed patterns of element partitioning. It is concluded that differences in the lateral-layerspreading rates of crystal faces bring about the sector zoning. The results have implications for the use of trace element partition coefficients in the modelling of fractionation processes.

S-type granitoids and an I-S line in the Peninsular Ranges batholith, southern California

Peraluminous to metaluminous (S-type) granitoids occur in the central and eastern parts of the Peninsular Ranges batholith, southern California, as highly deformed plutons and anatectic migmatites within metapelitic prebatholithic wallrocks of upper amphibolite fades. The granitoids overlap in age with, and have undergone a structural history similar to that of, the Early Cretaceous I-type granitoids that occur mainly to the west. Initial 87 Sr/ 86 Sr ratios greater than 0.7079 and δ 18 O greater than 11.8 indicate a significant metasedimentary component in the source rocks of the S-type granitoids. An I-S line that defines the westernmost extent of these rocks lies close to gravity, age, isotopic, and prebatholithic wallrock contrasts within the batholith, and together these features probably mark a fundamental change in the Mesozoic crust. If the western part of the batholith represents an Upper Jurassic and Lower(?) Cretaceous accreted tectonostratigraphic terrane, then the zone that contains these features may represent a terrane boundary. We propose that the S-type granitoids and associated migmatites developed at midcrustal levels as a result of collision and crustal thickening, and rose as diapirs, some dragging an envelope of migmatite and wallrock with them. The zone of intense metamorphism and deformation that now forms the axis of the batholith has been uplifted relative to the western part of the batholith, possibly as a consequence of unloading.