Petrography and geochemistry of late- to post-Variscan vaugnerite series rocks and calc-alkaline lamprophyres within a cordierite-bearing monzogranite (Sierra Bermeja Pluton, southern Iberian Massif)
Jon Errandonea-MartinFernando Sarrionandia-Ibarra EguidazuM. Carracedo‐SánchezJosé Ignacio Gil IbarguchiLuis Eguíluz Alarcón
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Abstract:
The Sierra Bermeja Pluton (southern Central Iberian Zone, Iberian Massif) is a late-Variscan intrusive constituted by cordierite-bearing peraluminous monzogranites. Detailed field mapping has allowed to disclose the presence of several NE–SW trending longitudinal composite bodies, formed by either aphanitic or phaneritic mesocratic rocks. According to their petrography and geochemistry these rocks are categorized as calc-alkaline lamprophyres and vaugnerite series rocks. Their primary mineralogy is characterized by variable amounts of plagioclase, amphibole, clinopyroxene, biotite, K-feldspar, quartz and apatite. Broadly, they show low SiO 2 content (49–56wt.%), and high MgO+FeO t (10–17wt.%), K 2 O (3–5wt.%), Ba (963–2095ppm), Sr (401–1149ppm) and Cr (87–330ppm) contents. Field scale observations suggest that vaugneritic rocks and lamprophyres would constitute two independent magma pulses. Vaugneritic dioritoids intruded as syn-plutonic dykes, whereas lamprophyres were emplaced after the almost complete consolidation of the host monzogranites. In this way, vaugnerite series rocks would be an evidence for the contemporaneity of crustal- and mantle-melting processes during a late-Variscan stage, while lamprophyres would represent the ending of this stage.Keywords:
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There are many acidic intrusions of varying age in the northern and southern zones of the eastern Pontides, NE Turkey, and the Sar›han Granitoid is one of these. The Maastrichtian Sar›han Granitoid was emplaced into the pre-Permo—Carboniferous Pulur Massif, generally comprising medium-grade metamorphic rocks; the Liassic Hamurkesen Formation, that begins with the Dikmetafl conglomerate and continues upward into volcano- sedimentary rocks; and the Malm—Lower Cretaceous Hozbirikyayla Formation, comprising limestone and sandy limestones. The Sar›han Granitoid crops out in an area of approximately 40 km 2 , has an ellipsoidal shape, and comprises mainly quartz monzodiorite, granodiorite and lesser quartz diorite. The pluton contains volcanic and silicified limestone xenoliths and dioritic mafic microgranular enclaves. The plutonic rocks show medium-grained, poikilitic, monzonitic, anti-rapakivi and sometimes myrmekitic textures, and contain 43—64% plagioclase, 6—18% orthoclase, 10—29% quartz, 5—20% hornblende, 1—85% biotite, 1—6% opaque oxides, accessory amounts of apatite, titanite and zircon, and secondary phases of calcite, chlorite and sericite. Some textures may suggest magma mixing, whereas the presence of mafic microgranular enclaves indicates that magma-mingling processes were operative in the evolution of the pluton. The pluton has 65—67% SiO2, 1.4—3.1% MgO, 4.1—5.5% Na2O and <1 K2O/Na2O. Generally, the pluton is I-type, metaluminous and has characteristics of cafemic-group granitoids, suggesting a hybrid source derived by mixing of sialic and mantle sources. The pluton has calc-alkaline composition and is characterised by a calc-alkaline granodiorite-series trend. TiO2, Al2O3, FeO, Fe2O3, MnO, MgO, CaO, P2O5, Ba and Ni decrease whereas Na2O, K2O, Rb and Nb increase with increasing SiO2 content. These geochemical variations indicate the importance of fractional crystallisation, which was mainly controlled by plagioclase and hornblende. However, some irregular variations in major and trace elements may be results of magma mixing. Zn, Rb, Sr, La, Pb, Th and Zr show enrichment whereas Ce, Cr and Ni exhibit depletion compared to continental crust values, resembling those of volcanic-arc granitoids. With regard to discrimination of tectonic setting, the pluton represents pre-plate collision volcanic-arc granitoids. The 87 Sr/ 86 Sr ratio (0.70504) of the pluton also indicates a hybrid magma which likely was derived by mixing of a mantle source with a crustal component. Field observations suggest a stoping type of ascent and emplacement style.
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The Panticosa pluton is one of multiple granitic plutons cropping out in the Pyrenean Axial Zone, which corresponds to the Palaeozoic core of the Pyrenees. Mafic dikes cut both the Panticosa pluton and its Devonian metasedimentary country rocks. According to their mineralogy and mineral composition, these dikes can be classified into two different groups, with little textural variations: a calc-alkaline group and an alkaline group. The calcalkaline rocks (spessartites and calc-alkaline diabases) include calcic to sodic Pl, slightly Ti-rich Amp and substantial Qz. In contrast, the alkaline rocks (camptonites and alkaline diabases) are composed of calcic Pl, Ti-rich Cpx, Ol pseudomorphs and inherited constituents (megacrysts and enclaves). The composition of Pl and the main ferromagnesian phase (Amp in calc-alkaline group and Cpx in alkaline group) is coherent with the geochemical affinity assigned to both groups. The megacrysts in the alkaline group are predominantly Krs and Ts, with a deep-origin according to thermobarometric calculations. A detailed mineral chemistry study of the phases observed inside the enclaves is necessary to better comprehend the petrogenesis of the magmas which generated these alkaline rocks.
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The Upper Precambrian Terra Nova Pluton, situated 550 km inland from Recife, Brazil, is 220 km 2 in area and intrudes deformed metasedimentary rocks of the Pianco-Alto Brigida Mobile Belt. The pluton shows complex petrological relationships. It consists of subalkaline quartz-monzonites and quarts-syenites, and the major minerais are K-feldspars, albite, hornblende, and quartz. The pluton is intermediate in composition (Si0 2 =58.9-65.6 wt %; MgO=0.9-3.7 wt % ) and is dominantly potassic (K 2 0=3.3-5.6 wt %; K 2 O/Na 2 O=0.9-1.8). Ba (up to 2,300 ppm) and Sr (up to 1,100 ppm) are abundant in the rocks, and LREE are enriched relative to HREE (La N /Lu N = 25.6-43 .2). There is no significant Eu anornaly. Rounded autoliths with in the pluton are similar, but more maflc in composition (Si0 2 = 54.6-57.5 wt %; MgO = 4.9-6.4 wt %). A suite of dykes cut pluton and the surrounding country rocks. These dykes are varied in composition, encompassing most of the chemical range shown by the pluton and associated auto liths. The dykes are holocrystalline, peralkaline, and strongly enriched in both K 2 0 (K 2 0 = 5.3-11.4 wt %) and Ba (Ba = 2,400 ppm-l0,500 ppm), which are considered to be magmatic abundances. The dykes have similar REE and o ther trace elements and ratios to the auto liths and plutonic rocks, and the dykes and the pluton are thought to be chemically related. The Terra Nova Pluton records the fractionation of mantle-derived ultrapotassic magma from mafic to intermediate compositions.
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The 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.
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