Two-mica Monzogranite Research Articles

Petrogenesis and tectonic implications of Indosinian granitoids from Western Qinling Orogen, China: Products of magma-mixing and fractionation

The Western Qinling Orogen (WQO) is characterized by voluminous distribution of Indosinian granitoids, the formation of which provides an important window to unravel the geochemical and geodynamic evolution and associated metallogeny. Here we investigate a group of intrusions termed “Five Golden Flowers” based on petrological, geochemical, zircon U–Pb geochronological and Lu–Hf isotopic studies on the granitoids and their mafic microgranular enclaves (MMEs). Our results show that these intrusions are genetically divided into two types, namely, magma-mixing and highly fractionated. The Jiaochangba, Lujing, Zhongchuan, and Luchuba granitoids are biotite monzogranites (220 ​± ​0.8 ​Ma to 217 ​± ​2.6 ​Ma) with abundant coeval MMEs (220 ​± ​1.1 ​Ma to 217 ​± ​2.7 ​Ma). The rocks contain moderate to high SiO2, high MgO, Rb, Sr, Ba, and Th contents, but low TiO2, P2O5, and Sc values, A/CNK of <1.1, and a range of εHf(t) values of −11.7 to +2.23 with corresponding TDM2 values of 1967–1228 ​Ma. The MMEs possess K-feldspar megacrysts, abundant acicular apatites, and show lopsided textures. They have lower SiO2, Al2O3, and Th contents, but higher MgO, TiO2, and Sc, with εHf(t) values of −18.0 to +3.18 and TDM1 of 849–720 ​Ma. The data indicate that the MMEs were derived from a magma sourced from the enriched lithospheric mantle. We suggest that these host granitoids were produced by partial melting of late-Paleoproterozoic to early-Mesoproterozoic lower crust with the involvement of Neoproterozoic SCLM-derived mafic magmas. The Baijiazhuang pluton is dominantly composed of leucogranite (muscovite granite and two-mica monzogranite, 216 ​± ​1.5 ​Ma) without MMEs. The rocks are peraluminous with high A/CNK (1.06–1.27). Compared with the other four granitoids, the Baijiazhuang leucogranite shows higher SiO2 content, markedly lower concentrations of TiO2, MgO, Al2O3, CaO, and Fe2O3T, and lower LREE/HREE and (La/Yb)N values. These leucogranites are also rich in Rb, Th, and U, and display marked depletions in Ba, Sr, Ti, and Eu, indicating that they experienced significant fractionation. Zircon εHf(t) values (−10.2 to −3.27) and TDM2 (1868–1424 ​Ma), as well as the Nb/Ta and K2O/Na2O values are similar to the other four granitoids, indicating that they are likely to have been derived from a similar source; with sediments playing only a minor role in the magma generation. The low contents of Yb and Y suggest that their partial melting was controlled by garnets and micrographic texture of K-feldspar reflects high-temperature melting through undercooling. Based on the above features, we infer that the Baijiazhuang leucogranite likely represents the product of high degree fractionation of the I-type biotite monzogranite magma which generated the other four granitoids at relatively high temperatures, within magma chambers at mid-crust depths. We propose that the granitoid suite was formed in the transitional setting from syn- to post-collision during the collisional orogeny between the SCB and NCB, following break-off of the subducted South China Block lithosphere during 220–216 ​Ma.

Geochronology, geochemistry and Hf isotopic composition of Late Cretaceous Laojunshan granites in the western Cathaysia block of South China and their metallogenic and tectonic implications

Sn-, In- and W-rich granites are identified in Laojunshan area in the western Cathaysia block, South China, including coarse-grained two-mica monzogranite (CMG) and fine-grained two-mica monzogranite (FMG). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb dating of CMG and FMG are from 85.6 ± 0.8 Ma to 90.37 ± 0.7 Ma and 89.9 ± 1.4 Ma, respectively. All samples have relatively high total alkalis contents (5.74 wt% to 9.73 wt%) and Al saturation indices (0.95 to 1.79), indicating that most of granites belong to the strongly peraluminous granites. The granites are enriched in P, Rb, Cs, Sn, Li, Be, and U, and are depleted in Ti, Mg, Co, Ni, Sr, Ba, Zr, and Hf. The fractional crystallization of plagioclase and K-feldspar was the principal process of magmatic differentiation that controlled Rb, Sr, Ba and Eu concentrations. While rare earth elements were fractionated by accessory minerals such as apatite, monazite or ilmenite. These geochemical features suggest that most of rocks are highly fractionated S-type granites. The εHf(t) values of the zircon samples from CMG and FMG range from −12.88 to −1.19, with the two-stage Hf model ages ranging from 1.2 Ga to 1.9 Ga. The geochemical and isotopic data show that the Laojunshan granites are derived from a crystalline basement that has metamorphosed pelitic rocks from the Mes-paleoproterozoic continental crust. The upwelling of the asthenosphere caused by a strong lithospheric extension led to the partial melting of the metamorphosed pelitic rocks to form widespread Late Cretaceous S-type granites in the Laojunshan area. Lithospheric extension and Cretaceous magma in SE Yunnan are probably related to the Palaeo-tethys ocean crustal delamination, which subducted beneath the Indochina block.

Origin of Early Creceouscalc-alkaline granite, Taxkorgan: Implications for evolution of Tethys evolution in central Pamir

The Pamir plateau may have been a westward continuation of Tibet plateau. Meanwhile, the Rushan-Pshart suture is correlative to the Bangong-Nujiang suture of Tibet, and the Central Pamir is the lateral equivalent of the Qiangtang Block. We present the first detailed LA-ICPMS zircon U-Pb chronology, major and trace element, and Lu-Hf isotope geochemistry of Taxkorgan two-mica monzogranite to illuminate the Tethys evolution in central Pamir. LA-ICPMS zircon U-Pb dating shows that two-mica monzogranite is emplaced in the Cretaceous (118 Ma). Its geochemical features are similar to S-type granite, with enrichment in LREEs and negative Ba, Sr, Zr and Ti anomalies. All the samples show negative zircon eHf(t) values ranging from −17.0 to −12.5 (mean −14.5), corresponding to crustal Hf model (TDM2) ages of 1906 to 2169 Ma. It is inferred that these granitoids are derived from partial melting of peliticmetasedimentary rocks analogous to the Paleoproterozoic Bulunkuole Group, predominantly with muscovite schists component. Based on the petrological and geochemical data presented above, together with the regional geology, this work provides new insights that Bangong-Nujiang Ocean closed in Early Cretaceous(120–114 Ma).

Geochronology and source of the rare-metal pegmatite in the Mufushan area of the Jiangnan orogenic belt: A case study of the giant Renli Nb–Ta deposit in Hunan, China

The Jiangnan orogenic belt hosts many rare-metal deposits that exhibit close ore-forming ages and similar geological backgrounds and is an important Li–Be–Nb–Ta concentration belt located in eastern China. However, the metallogenic mechanism and regularity of these rare-metal deposits have not yet been reported in detail because of the lack of accurate geochronological data and systematic studies on the sources of metallogenic materials. The giant Renli rare-metal deposit, located at the south margin of the Mufushan batholith in the central Jiangnan orogenic belt, is one of the few high-grade granitic-pegmatite-type Nb–Ta deposits in China. Columbite–tantalite U–Pb dating has been developed as a more robust method when compared with the traditional dating methods to constrain the timing of rare-metal mineralization. Six columbite grains, obtained from the microcline–albite pegmatite of the Renli deposit, exhibit a concordant laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb age of 133.0 ± 2.6 Ma, which denotes the Nb–Ta mineralization age. The zircon grains from the columbite-bearing microcline–albite pegmatite subjected to different degrees of alteration by the hydrothermal fluids yield a concordant LA–ICP–MS U–Pb age of 131.2 ± 2.4 Ma, which can be considered to be the age of newly magmatic–hydrothermal growth and recrystallization under the transitional magmatic–hydrothermal conditions observed in case of the Renli deposit. Furthermore, the zircon grains from the host biotite monzogranite yield concordant LA–ICP–MS U–Pb ages of 140.7 ± 0.7 and 140.3 ± 0.7 Ma, whereas the zircon grains from the host two-mica monzogranite yield a concordant LA–ICP–MS U–Pb age of 138.3 ± 0.3 Ma, denoting the intrusion timing of two types of host Mufushan granites of the pegmatite dykes. Finally, the zircon grains from two schist samples of the Lengjiaxi Group yield concordant LA–ICP–MS U–Pb ages of 825.9 ± 5.6 and 834.4 ± 5.3 Ma, constraining the maximum age for the termination of deposition. These ages constitute a clear intrusion-cooling-hydrothermal time series with respect to the Renli deposit.Zircon can retain the original magmatic isotopic composition and is extensively used to trace the petrogenetic processes because it is quickly crystallized from magma. The Mufushan granite and rare-metal pegmatite exhibit consistent negative eHf(t) values (mean eHf(t) values of −7.2 and −7.3, respectively), and their two-stage model ages exhibit similar distribution patterns when compared with that exhibited by the Lengjiaxi Group. Furthermore, the Mufushan granite shows the typical geochemical characteristics of the crustal-derived granite. Thus, the rare-metal pegmatite and host granite of the Renli deposit were concluded to be sourced from the partial melting of the Lengjiaxi Group metasedimentary strata.The compressive torsional nature of the Gajiang fault zone, widespread occurrence of the S-type granite, and absence of basaltic rocks suggest that the central-to-eastern part of the Jiangnan orogenic belt was in a subduction setting during the Early Cretaceous. Thus, this economically important Early Cretaceous (ca. 133–129 Ma) rare-metal metallogenic belt was formed owing to the development of the Jiangnan orogenic belt, and the thick Lengjiaxi Group metasedimentary strata provided the metallogenic material source.

Petrogenesis and tectonic setting of the Wulong two-mica monzogranite on Liaodong Peninsula, NE China: Constraints from zircon U-Pb and Hf-O isotopic data

Petrogenesis and tectonic setting of the Wulong two-mica monzogranite on Liaodong Peninsula, NE China: Constraints from zircon U-Pb and Hf-O isotopic data

Interaction of B-rich supercritical magmatic fluids with granite: first report of dumortierite in Larderello, Italy

In a peraluminous two-mica monzogranite cored at 4.5 km depth in well Radicondoli 29, dumortierite occurs together with andalusite, Li-rich tourmaline, fluorite and fluorapatite. The average composition is SiO2: 30.9, Al2O3: 58.6, FeO: 0.44, MgO: 0.47, CaO: 0.02, F: 0.15. Dumortierite crystallized from supercritical magmatic fluids at T =520-620°C, P = 100 ± 30 MPa. Synthetic fluid inclusions yielded T = 510 ± 10°C, P = 42 ±3 MPa at 2.9 km depth in well Venelle 2. All data suggest present-day T = 450-550°C, P = 40-80 MPa. The supercritical magmatic fluids were hypersaline brines with ~ 30 wt % LiCl and up to 2.4 wt % F, extreme contents that can be found only in pegmatites, aplites and leucogranites. Larderello granites derived from partial melting of a lower crust made up of interlayered metasediments and amphibolites. Extensive melting took place in the lower crust during the last 10 Ma owing to extensional tectonics, lithosphere thinning, roll back or break down of the west merging Adria plate and swelling of the asthenosphere below the western side of the Apennines.

The strongly peraluminous A-type granites of the Characato suite (Achala batholith), Sierras Pampeanas, Argentina: Evidence of Devonian-Carboniferous crustal reworking

The largest igneous body in the Sierras Pampeanas (the Achala batholith) is composed by several magmatic intrusions. How many plutons form this batholith and its intrusive history remains unclear. The emplacement of the batholith began at ca. 380 Ma; the Characato suite, in the northern extreme of the body, represents a relatively young intrusive phase in its construction (ca. 360 Ma). This suite, composed mainly of two-mica monzogranites, correspons to a strongly peraluminous A-type magma (A/CNK > 1.1) with fractionated compositions (high Rb/Sr and Rb/Ba). The parental magma corresponds to a high temperature melt generated by biotite dehydration melting, which was then differentiated to form the Characato monzogranites. These monzogranites have crustal Nd and Srisotope signatures (εNdt −6.0 to −6.1 and 87Sr/86Srt 0.7074 to 0.7131) and abundant inherited zircons (ca. 90%). The inherited ages are mainly Early Cambrian, and to a lesser extent Mesoproterozoic, Neoproterozoic and Ordovician, all they recognized in the basement of the Sierras de Córdoba. The Nd-isotope composition, added to the inherited zircon ages of the Characato suite, indicate that these magmas derived from partial melting of a heterogeneous crust composed by the S-type Pampean granites and migmatites, Famatinian granitoids, and probably the low-to medium-grade metamorphic rocks of the Puncoviscan Series. The high variability and positive εHft values of the inherited Cambrian zircons found in the Characato suite could have been transferred from the Neoproterozoic grains of the Puncaviscana Series to the Early Cambrian S-type granites and migmatites during the Pampean orogeny.

Petrogenesis of the Zheduoshan Cenozoic granites in the eastern margin of Tibet: Constraints on the initial activity of the Xianshuihe Fault

The Zheduoshan Miocene granitic pluton is exposed at the eastern margin of Tibet and along the strike-slip Xianshuihe Fault, and is the product of syn-tectonic magmatism closely related to this fault. This paper is focused on the petrogenesis of different granitic lithological units in the Zheduoshan composite intrusion, and the results of geochronology and lithology show that the Zheduoshan Miocene granitic pluton is incremental assembly by three stages of granitic magma influx and growth, represented by fine-grain biotite granite at 18.0 Ma, corase-grain and porphyraceous biotite monzogranite at 16.0 Ma and medium-grain two-mica monzogranite at 14.0 Ma. Combining with the geochemical signatures, these granitic rocks have high intial 87Sr/86Sr ratios, enriched Nd and Hf isotopic compositions, revealing that the sources of these granitic rocks are metabasatic rocks for fine-grain biotite granite, greywackes for coarse-grain biotite monzogranite and medium-grain monzogranite. These granites have high Sr/Y ratios, revealing that these granitic magma form at high pressure condition. The Sr/Y ratios and calculated crystallization pressure gradually decreased, implying the pressure gradually decreasing with the formation of these three stages of granites, which is probably caused by the tectonic mechanism transition from compression to strike-slip extension during the generation of these granites at 18.0–14.4 Ma. This tectonic mechanism change implied the initial activity of Xianshuihe Fault at least before 14.4 Ma.

Insights into petrogenesis of the Jálama pluton (Central Iberian Zone, western Spain)

ABSTRACTThe Jálama pluton (JP) is a Variscan peraluminous granitoid that intruded into low-grade metasediments from the Central Iberian Zone (CIZ). It comprises a sillimanite-bearing two-mica monzogranite in the inner zone, followed by a tourmaline-bearing two-mica monzogranite, and a marginal tourmaline-muscovite leucogranite in the northern half of the pluton. Microgranitoid enclaves and metasedimentary xenoliths occur locally in monzogranites. The change in rock type from the central monzogranite to the marginal leucogranite corresponds to decreasing TiO2, MgO, FeO, CaO, Sr, Ba, Zr, and ΣREE, and increasing SiO2, Na2O, P2O5, Rb, Li, Cs, Ta, Sn, and W. Fe/(Fe+Mg) ratios in biotite, muscovite and tourmaline increase with increasing Fe/(Fe+Mg) in bulk rock, suggesting an important control of the bulk-rock composition on mineral chemistry. The high peraluminosity, the low CaO and high P contents, as well as the similarity of ε(Nd)300 values in both the granites and metasediments of the southern CIZ constitute strong evidences for a crustal origin of the granite suite, probably by melting of these metasedimentary rocks. Field and petrographic observations, together with mineralogical and geochemical data, suggest that assimilation and mingling/mixing acted in concert with fractional crystallization during the formation of the JP. These processes may also have been important in the evolution of other granitoids from this region.

Origin of the Late Jurassic to Early Cretaceous peraluminous granitoids in the northeastern Hunan province (middle Yangtze region), South China: Geodynamic implications for the Paleo-Pacific subduction

The Late Mesozoic granitic belt in the northeastern Hunan province (situated in the south of the middle Yangtze region) represents the western front of the large magmatic province of SE China. In order to determine their ages and petrogenesis, we carried out zircon U–Pb dating, Hf isotope and whole-rock geochemical analyses for four granitic plutons, namely Taohuashan, Dayunshan-Mufushan, Wangxiang and Lianyunshan. Our SIMS zircon U–Pb ages, together with previously published data, reveal that the magmatic activities in this area can be roughly subdivided into three phases at 151–146Ma, 132–127Ma and ca. 117Ma, and the Dayunshan-Mufushan batholith therein is a composite pluton. These four plutons are mainly composed of weakly to strongly peraluminous biotite or two-mica monzogranites, with a minor amount of biotite granodiorites. Their geochemical features are similar to S-type as well as fractionated S-type granites, with enrichment in LREEs and negative Ba, Sr, Nb, P and Ti anomalies. All samples show negative zircon εHf(t) values ranging from −12.5 to −3.6, corresponding to crustal Hf model (TDMC) ages of 1.4–2.0Ga. It is inferred that these granitoids were derived from partial melting of metasedimentary rocks analogous to the Neoproterozoic Lengjiaxi Group, predominantly with psammitic component. Fractional crystallization probably played an important role in the magma evolution, while input of mantle-derived magma was insignificant. Combined with other geological evidence, our new data allow us to propose that the Cretaceous (132–127Ma and ca. 117Ma) magmatism might be response to episodic slab rollback of the Paleo-Pacific plate, while the early-stage (151–146Ma) magmatism that overlapped the epilogue of Jurassic magmatic flare-up and subsequent magmatic quiescence probably foreshadowed the transformation from foundering of a subducted flat-slab to slab rollback. Alternatively, slab foundering after a SE-directed intracontinental subduction in the central SCB cannot be ruled out for geodynamic interpretation of the Jurassic magmatism in SE China.

Timescales and mechanisms of batholith construction: Constraints from zircon oxygen isotopes and geochronology of the late Variscan Serre Batholith (Calabria, southern Italy)

The late Variscan Serre Batholith in central Calabria represents the middle portion, c. 13-km-thick, of a tilted crustal section continuously exposed from lower-crustal granulites to upper-crustal phyllites. The batholith is zoned, consisting of several granitoid types that were emplaced at depths ranging from c. 23 to c. 6km. Deep, strongly foliated quartz diorites and tonalites were emplaced into migmatitic metapelites, the intermediate level granitoids are weakly foliated to unfoliated porphyritic granodiorites and monzogranites, and the shallowest bodies are two-mica granodiorites and granites, grading upward to biotite ± amphibole granodiorites, emplaced into paragneisses and phyllites. Five samples, representative of the main granitoid types in terms of both composition and emplacement depth, have been dated by SHRIMP. Zircon from a lower-crustal quartz diorite gave an emplacement age of 297.3±3.1Ma; two middle-crustal strongly peraluminous K-feldspar megacrystic granites were emplaced at 296.1±1.9Ma and 294.9±2.7Ma; a middle- to upper-crustal two-mica monzogranite was emplaced at 294.2±2.6Ma and finally, an upper-crustal weakly peraluminous granodiorite from the batholith roof was emplaced at 292.2±2.6Ma. These results are consistent with the development of the batholith by incremental multipulse overaccretion, each granitoid body being in a dominantly rigid state before intrusion of the next, with little or no possibility of magma mixing. The difference in age between the oldest and youngest granitoids, from the batholith floor and roof, respectively, is 5.1±4.0Ma, providing an upper limit of about 9Ma on the time taken for batholith construction. The presence in the c. 296–294Ma granitoids of c. 305–302Ma anatectic zircon with varied Th/U and oxygen isotope compositions indicates a time interval of 8–9Ma between incipient partial melting in the lower crust and magma emplacement in the middle crust. The emplacement of the first granitoid bodies into the top of the lower crust was controlled tectonically by the activation of a deep-seated shear zone. The shallowest granodiorites were emplaced during the waning stages of the shear zone activity, producing late- to post-tectonic contact metamorphism in the upper-crustal phyllites and mylonitic paragneisses.

Syn-collisional granitoids in the Qilian Block on the Northern Tibetan Plateau: A long-lasting magmatism since continental collision through slab steepening

In this paper we present a new model that can explain the large zircon age spectrum of ~510 – 420Ma within a single sample from the Gangcha (Gcha) biotite granodiorite and the Huangyuan (HY) two-mica monzogranite on the northern Tibetan Plateau. The large age spread recorded in zircons is characteristic of granitoid samples from the studied region, which is best explained by the long-lasting magmatism since the onset of continental collision at ~500Ma, followed by slab steepening and the ultimate slab break-off at ~450Ma. These granitoids have a large major and trace element compositional variation, but limited initial Sr (ISr[450]=0.709 to 0.715), Nd (ԐNd[450]=−6.5 to −3.7), Hf (ԐHf[450]=−4.3 to 1.5) and Pb (206Pb/204Pb[450]=17.70 to 17.17; 207Pb/204Pb[450]=15.60 to 15.69; 208Pb/204Pb[450]=38.04 to 38.73) isotopic variation. The small negative whole rock ԐNd[450] and ԐHf[450] values are most consistent with the granitoid source being dominated by subducted seafloor materials. The inherited zircons with large negative ԐHf[450] values (e.g. −50) are indicative of input from the lower continental crust and subducted sediments. The correlated variations among major elements, trace elements and radiogenic isotopes are best interpreted as reflecting melting-induced mixing of a compositionally heterogeneous source with superimposed effect of varying extent of fractional crystallization and crustal assimilation. The inherited zircons of Palaeo-Proterozoic age and the Archean crustal model ages signify the involvement of ancient basement rocks.

Spatial association of Neoproterozoic continental arc I-type and post-collision A-type granitoids in the Arabian–Nubian Shield: The Wadi Al-Baroud Older and Younger Granites, North Eastern Desert, Egypt

The Neoproterozoic basement of Wadi Al-Baroud area located at the northern Eastern Desert (ED) of Egypt, at the northernmost segment of the Arabian–Nubian Shield (ANS), is comprised of two different granite suites. A large batholith ascribed to the Older Granite suite, extends across the boundary between the northern and central ED, and is intruded by two isolated plutons of the Younger Granite suite. The Older Granite suite includes gray-colored, massive to gneissose, granodiorites to tonalites typically containing microgranular mafic enclaves. These are calc-alkaline, magensian, metaluminous I-type granitoids, with high Sr contents, and depleted in Rb, Nb, Y and REE. The Younger Granite suite plutons are pink to red, biotite and two-mica monzogranites. These are peraluminous A-type granites exhibiting a high-K calc-alkaline nature, and varying between ferroan and magnesian type granites. The A-type granites of the Younger Granite suite are enriched in Ga, Y, HFSE and REE elements, and depleted in the LILE elements Ba, Sr and Rb and transition metals Cr, Ni, Co, Sc and V. Magmatic saturation temperatures indicate early crystallization of apatite at high temperature in the metaluminous I-type Older Granite suite, while in the peraluminous A-type Younger Granites its crystallization occurs later after separation of zircon and monazite. The plutons of the Younger Granite suite were generated during the post-collisional stage of the northern ANS, following collision between the juvenile ANS crust and the pre-Neoproterozoic continental blocks of west Gondwana. The emplacement of the Older Granite suite took place earlier, within a normally mature continental arc prior to the collision. These pre-collision granitoids evolved through assimilation-fractional crystallization processes from mantle-derived parental magmas, which have interacted with crustal materials during ascent and storage. The post-collisional Younger Granite suite seems to have been derived by high degree, partial melting of metasedimentary sources, particularly psammitic and pelitic metasediments.

Geochemistry and zircon U–Pb–Hf isotopic systematics of the Ningshan granitoid batholith, middle segment of the south Qinling belt, Central China: Constraints on petrogenesis and geodynamic processes

The Ningshan Granitoid Batholith (NGB) is located in the middle segment of the South Qinling Belt (SQB) section of the Qinling orogenic belt, in Shaanxi Province, China, and consists of the Laocheng, Yanzhiba and Lanbandeng intrusions. The Laocheng intrusion is dominated by quartz diorites, granodiorites and monzogranites, with the Yanzhiba intrusion being granodiorite dominated, and the Lanbandeng intrusion predominantly consisting of two-mica monzogranites. LA–ICP–MS zircon U–Pb isotopic dating indicates that the NGB formed during two distinct episodes of magmatism, with an earlier episode at 222–216Ma and a later episode at ∼210Ma. Whole-rock geochemistry and geochronology data have allowed us to divide the NGB into three groups, with Group #1 rocks consisting of Laocheng intrusion quartz diorites and granodiorites, and Yanzhiba intrusion granodiorites that were produced in the early magmatic episode. Group #1 samples have high-K calc-alkaline compositions, and are characterized by high Mg# values, high Sr, Cr, and Ni concentrations, and high (La/Yb)N and Sr/Y ratios, but low Yb and Y concentrations, implying that these rocks were formed from a mixed magma derived from melting of Neoproterozoic crustal basaltic rocks and magmas sourced from a region of depleted mantle. Group #2 rocks are composed of the mid-K calc-alkaline monzogranites in the Laocheng intrusion that were formed in the later magmatic episode, and are characterized by lower Mg# values, and lower Cr, Ni, and Co concentrations than the Group #1 samples, but with high Sr concentrations and (La/Yb)N and Sr/Y ratios, low Y and Yb concentrations, negligible Eu anomalies, and a significant range in zircon εHf(t) values. The highest zircon εHf(t) values of the Group #2 samples plot close to the depleted mantle evolutionary line, indicating that these rocks were derived from a mixed magma with both crustal- and mantle-derived components. Group #3 samples consist of high-K calc-alkaline two-mica monzogranites of the Lanbandeng intrusion that were formed during the later magmatic episode. These rocks are characterized by high SiO2, Al2O3, and K2O concentrations, with low MgO and Sr concentrations, low (La/Yb)N and Sr/Y ratios, Nb, Ta, P and Ti depletions, and significantly negative Eu anomalies, implying that the magmas from which these rocks formed were derived from partial melting of sedimentary rocks and minor amount of basalt. The early magmatic episode associated with formation of the NGB occurred in a continental arc environment at 222–216Ma, with ∼210Ma magmatism during a geodynamic transition from syn-collisional to post-collisional tectonics across the SQB.

X-ray study of potassium feldspars from different granitoid types and gneisses of Papuk Mt. (Slavonia, Croatia)

Potassium feldspars from different granitoids and gneisses of Papuk Mt. (Slavonia, Croatia) have been investigated by X-ray powder diffraction. Diffraction patterns classically observed as well as patterns calculated by Rietveld refinement were compared and discussed. Triclinicity was calculated according to GOLDSCHMIDT & LAVES (1954) while the structural state of the feldspars was determined using the methods of KROLL & RIBBE (1983) and GODINHO & JALECO (1973). Results showed that the type of potassium feldspar depend on the investigated host-rock, indicating variation in the structural state from orthoclase, intermediate microcline to highly ordered microcline. Potassium feldspar megacrysts in biotite-granodiorites and monzogranites are intermediate microcline or orthoclase, while two-mica monzogranites contain low microcline. Gneisses contain low microcline and orthoclase in Brzaja Creek and low microcline in Djedovica Quarry. Classically observed and digital diffraction patterns calculated by the Rietveld refinement method produced comparable results and provided a very good correlation of the results obtained by different methods. High triclinicity values of feldspars from investigated granitoid and gneiss samples from Papuk Mt. (Slavonia, Croatia) are in accordance with a high Al content in the T1o site and their fully ordered state indicates a slow(er) cooling-rate. Low triclinicity values, an Al content in T1o site around 0.60 and ordering index smaller than 0.80 can be interpreted as a result of relatively fast(er) cooling which allowed lower ordering of the potassium feldspar.

Petrogenetic Appraisal of Early Palaeozoic Granitoids of Kinnaur District, Higher Himachal Himalaya, India

The Early Palaeozoic felsic magmatism of Kinnaur district is represented by more-or-less equivalent Akpa granitoids (477±29 Ma) and Rakcham granitoids (453±9 Ma), which form an integral part of the Central Crystalline Zone in the southern portion of the Tethys Himalayan Tectogen. Both, Akpa granitoids (AG) and Rakcham granitoids (RG), intrude the rocks of the Vaikrita Group. Substantially low magnetic susceptibility values (c = 0.016-0.187 × 10 −3 SI) of these Early Palaeozoic granitoids suggest their nature similar to as ilmenite series (reduced-type) granitoids emplaced in a syn-collisional tectonic setting. The modal compositions of AG mainly correspond to two-mica (muscovite-biotite) monzogranite belonging to the granitoids generated by crustal fusion. Based on mineral assemblage (Ms+Bt+Tur+Ap), associated skarn-type tungsten mineralization, occurrence of metasedimentary enclaves, and whole-rock geochemistry (SiO 2 = 70.75-72.95 wt.%, TiO 2 = 0.02-0.17 wt.%, K 2O/Na 2O = 1.05-2.32, molar A/CNK = 0.93 to 1.33, CIPW corundum 0.17 to 3.82 wt.%, Av. Sr = 304ppm, initial 87Sr/ 86Sr = 0.7206±0.00235), the AG can be typically characterized as peraluminous (S-type) granite as similarly noted for RG. Higher Rb/Sr ratio and initial 87Sr/ 86Sr = 0.737±0.002 of RG compared to that of AG however indicate involvement of relatively more crustal component in generation of RG. The AG is enriched in Ba and light rare earth elements (LREE). The chondrite normalized REE patterns of AG appear weakly fractionated (La N/Lu N= 1.06-6.43) and show pronounced negative Eu-anomaly (Eu N/Eu * = 0.03). The AG and RG represent typical S-type granite suites that largely evolved due to differential degree of anatexis (partial fusion) of sedimentary protoliths, prior to Caledonian but after the Pan-Indian thermal orogenic event.

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

RADARSAT data applications: radar backscatter of granitic facies, the Zaer pluton, Morocco

Radar backscatter in active microwave remote sensing is influenced by such physical properties of the ground as the surface roughness, shape and water contents, and varies with the wavelength and the angle of incident energy. Canada's RADARSAT is the first spaceborne radar which provides flexibility of incidence angle and ground resolution. Its imaging device is a C-band Synthetic Aperture Radar (SAR), which is particularly well suited for interpreting micro-roughness and soil conditions by the appropriate choice of imaging parameters. This article describes a test case in which quantitative RADARSAT backscatter data were related to spatial variations in a granitic pluton. The Zaer pluton, one of the several Hercynian granitoids in northwestern Morocco, consists of medium- to coarse-grained biotite granodiorite and two-mica monzogranite. These are highly weathered and the rock surfaces are largely covered with granitic soils of comparable mineralogical composition. The soils on the biotite granodiorite are richer in micaceous and clayey minerals than those overlying two-mica monzogranite. The variation in radar backscatter response, measured with calibrated RADARSAT data, is attributed to the different grain size and water content of the granitic facies. It is concluded that with RADARSAT data, the radar backscatter response differences can be highlighted for the purpose of facies mapping and geochemical interpretation.

Geochemistry and Genesis of the S-Type, Cordierite-Andalusite-Bearing Capillitas Batholith, Argentina

The Ordovician Famatinian-age magmatic cordierite-andalusite-bearing Capillitas batholith, in the Pampean Ranges of northwestern Argentina, encompasses a coarsely porphyritic to equigranular two-mica monzogranite with equigranular, fine-grained, late leucogranitic muscovite-rich facies. This batholith exhibits sharp, discordant contacts with low-pressure biotite-andalusite-cordierite schists of the La Cébila Formation, locally developing biotite-cordierite-muscovite-bearing contact hornfels aureoles. The two-mica monzogranite contains cordierite, andalusite, and sillimanite, although cordierite and andalusite are more abundant in the leucogranitic equigranular facies. These minerals are not homogeneously distributed and the three minerals are found together only locally. The presence of biotite-rich xenoliths with a high amount of anhydrous aluminum silicates (andalusite, sillimanite) and cordierite, exhibiting textures similar to those of the host monzogranites, suggest that, at least in part, they have been incorporated into the magma and reequilibrated. The pressure during emplacement was probably 4 kbar under near-solidus temperature, thus preserving the anhydrous aluminum-silicate stability under high H2O activity. The major-element data indicate a peraluminous calc-alkalic trend with compositional gaps. They attest to the existence of two distinct magma pulses. The chondrite-normalized REE patterns and multi-element spidergrams point to a probable origin by crustal (metasediment?) anatexis for both pulses. Peraluminous granitic magma cannot be a primary melt of metaluminous quartz-amphibolite, since there is a great geochemical homogeneity of all the granitoids. Igneous xenoliths are absent and the isotopic compositions of the granitoids correspond to those of metasedimentary sources. Both major and trace elements point to a collisional tectonic environment of an inner-continental magmatic arc.

Genesis of the two main types of peraluminous granitoids

Two-mica monzogranites to leucogranites and biotite-rich, cordierite-bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids. They can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation. Except for the rare muscovite-bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas, the majority of peraluminous granitoids are produced by partial melting of crustal rocks. Production of either muscovite-bearing granites or biotite-rich, cordierite-bearing granitoids does not depend only on the nature of the sources, but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced. Biotite-rich, cordierite-bearing granitoids are generated where mantle-derived magmas are injected into or have underplated crustal rocks; two-mica granites are generated where thickened crust is affected by major crustal shears or thrusts. Correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids.