Assimilation and diffusion during xenolith-magma interaction: a case study of the Variscan Karkonosze Granite, Bohemian Massif
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Massif
Xenolith
Anorthosite
Magma chamber
The Cretaceous-Palaeogene alkaline province, mainly located in the Kırşehir Block, extends E-W for some 300 km in Central Eastern Anatolia. The Dumluca, Murmana, Karakeban and Çaltı plutons constitute the easternmost part of the province. These plutons have introsive contacts with the Cretaceous Divriği ophiolitic melange and, except Karakeban, are associated with huge iron deposits. The Ypresian-Lutetian sediments unconformubly overly them. The four studied plutons belong to two completely different types of magmatic assocsiation, based on petrographic and geochemical (mainly major elements and REE) fetures. The dominant association comprises three plutons (Dumluca, Murmana, Karakeban) and is rather uniformly represented within each of them. These three plutons have a bimodal character evidenced by the coexistence of two groups of rocks, one mafic and the other felsic. The dominant felsic group, mainly composed of Na-rich and Ca-poor quartz monzonites and monzonites (Murmana, Dumluca), may also include syenites, quartz syenites, adamellites and granites (Karakeban). This group displays a cafemic alkaline over staurated trend, usually magnesian (Dumluca and Murmana) and common (Karakeban). The mafic group is mainly made up Na-rich and Ca-poor gabbros/diorites, monzogabbros/monzodiorites, rather often silica-undersaturated. This group represents a cafemic alkaline saturated to undersaturated trend either ferriferous (Karakeban) or magnesian (Dumluca) or or common (Murmana). Mafic dykes, cutting through the felsic rocks, belong to the same mafic group. These three plutons represent a composite alkaline association and the same type of association characterizes also each individual pluton. Field, petrographic and geochemical data suggest that the felsic group has not been derived from the mafic one by crystal fractionation, but that the two coevial groups may have interacted with each other. The subordinate association is represented by Çaltı pluton. This homogenous pluton, tonalitic and gronodioritic in composition, corresponds to a cafemic calc-alkaline association,with slight magnesian affinity. Such a coexistence of two quite different magmatic associations in the Divriği region has already been reported in the western part of Kırşehir Block. It shows that the alkaline character is restricted to only part of the plutons located in this block and may suggest that several successive magmatic events, related to quite different geodynamic conditions, occured in this domain. In this framework, the Dumluca, Murmana and Karakeban plutons are thougth to be derived from two different magma sources. One of them is mantle related mafic magma generated in a post-collisional lithospheric attenuation environment which accordingly caused to melt the lower parts of the continental crust forming the felsic magma source. This collisional event is attributed to the juxtaposition of the Pontide and Anatolide plates in the pre-Maastrictian time. The Çaltı pluton, representing different mineralogy and chemistry, should be solidified from the collision related and calcalkaline another hybrid magma source due especially to intruding the already obducted Divriği ophiolitic melange. Isotopic and geochronological data would be particularly helpful to better constrain this magmatic history.
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The Dongjiangkou granitic pluton is exposed in the South Qinling terrain between the Shang-Dan and the Mian-Lue suture zones.This granitic pluton generally contains abundant mafic enclaves of different types.Results of zircon U-Pb dating for a granite and two mafic enclaves by using the LA-MC-ICPMS technique show that the Dongjiangkou granitic pluton crystallized in 223Ma and mafic enclaves formed simultaneously in 222Ma.This magmatism of early Mesozoic constrains the time of tectonic transformation from compression to extension in the Qinling orogenic belt.Hf isotopic composition of zircons obtained by the LA-MC-ICPMS technique suggests that the granitic pluton resulted from a mixing-mingling process between mantle-derived mafic magma and crustal-derived granitic magma.The mafic magma originated likely from depleted mantle,while basement rocks of the South Qinling(or Yangtze) terrain contributed major material source for the granitic pluton.This magmatic activity is attributed to lithospheric delamination during the post-orogenic stage of the Qinling orogenic belt.
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Aztec Wash pluton, a 50 km[sup 2] intrusive complex in the northern Eldorado Mountains, was emplaced ca. 16 Ma (Faulds et al., 1990) during extension within the Colorado River Corridor. The pluton displays extreme compositional variability, ranging from olivine gabbro (ca. 50 wt% SiO[sub 2]) to highly evolved aplite (76% SiO[sub 2]). Most of the intrusion is medium grained, homogeneous granite (ca. 72% SiO[sub 2]), but 1/3 is highly heterogeneous and dominated by mafic to intermediate rocks; a 6 [times] 3km, N-S mafic zone almost bisects the pluton. Well-displayed magma mingling and late mafic and felsic dikes verify the coexistence of mafic and felsic melts. Hornblende barometry indicates that the entire exposed portion of Aztec Wash pluton was emplaced at very shallow depth (
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Sr/Rb dating, basement complex metamorphosed and intruded by igneous rocks before deposition of Grenville Group
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Geochemical and petrographic study of five calcalkaline Devonian plutons in the Northeast Kingdom batholith suggests that they were generated from similar sources but evolved differently. The modally homogeneous Willoughby granite (WG) and the Derby granodiorite (DG) are the most aluminous plutons and contain muscovite and biotite (+/- garnet). The West Charleston diorite (WCD) and the Nulhegan granodiorite (NG) are relatively mafic plutons containing pyroxene, hornblende, and biotite. The Echo Pond (EP) has a granodioritic core but is dioritic along its southern margin. Regular variations of major and trace elements are found in NG and in the relatively sodic DG. More erratic chemical variations are found for WG. The more mafic plutons have higher Sr and lower Rb, Nb and Ta compared to the more felsic WG and DG. Rare-earth patterns for all plutons show higher light than heavy rare-earths. However, the more mafic plutons are comparatively less enriched, have smaller Eu anomalies, and tend to have higher contents of heavy rare earths than the WG and DG. The felsic Willoughby pluton is the best candidate for an upper crustal minimum melt. A deeper origin for the more mafic plutons is likely. Significant differences in the rare earths and ferromagnesian elementsmore » indicate that the WG, DG, and EP cannot be related by a simple fractionation scheme to the more mafic plutons.« less
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Field relations and geochemistry indicate that Aztec Wash pluton had a complex, open-system history. The tilted pluton represents a 2.5 km thick chamber that was recharged with both felsic and mafic magma. The lower portion is highly heterogeneous, with mafic sheets; cumulates; hybrid rocks; mafic, felsic, and composite dikes; and sheets and pods of granite (heterogeneous [H] zone). The upper part is granite that is generally homogeneous in texture and geochemistry (granite [G] zone). At the base of the G zone, a discontinuous zone (buffer [B] zone) records interaction between the G and H zones. Complexity of the H zone makes detailed reconstruction of magma chamber history difficult, and the relatively homogeneous G zone appears to offer few clues about the evolution of the pluton or the interaction between the felsic and underlying more mafic magmas. Accessory mineral textures, zoning, and assemblages in the G zone, however, are far from homogeneous and provide clear evidence for fluctuating conditions that elucidates magma chamber history.
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The compositionally zoned Late Cretaceous Lamarck granodiorite, west of Bishop, hosts numerous mafic intrusions ranging from hornblende gabbro to mafic granodiorite. Frost and Mahood (1987) suggested from field relations that the Lamarck and the associated mafic plutons were co-intrusive. Contact relations between the Lamarck host and the mafic intrusions are variable (sharp to diffuse) and in places suggest commingling. In order to constrain the intrusive relationships between the Lamarck and its associated mafic plutons, the authors have analyzed feldspars from the Mt. Gilbert pluton and the Lamarck granodiorite to see if feldspar compositions in the Mt. Gilbert overlap those in the Lamarck host and determined U-Pb zircon ages for the Mt. Gilbert and Lake Sabrina plutons to see if they have the same age as the Lamarck granodiorite. Feldspars from the Lamarck granodiorite are normally zoned and range compositionally from An[sub 38--32]; those in the Mt. Gilbert diorite are also normally zoned but range compositionally from An[sub 49--41] and do not overlap the Lamarck host. Four to five zircon fractions from each pluton were handpicked and dated using U-Pb methods. The Mt. Gilbert mafic diorite has a concordant age of 92.5 Ma and the Lake Sabrina diorite has a concordantmore » age of 91.5 Ma. Ages for the two plutons overlap within error, but multiple fractions from each suggest that the Lake Sabrina pluton is slightly younger than the Mt. Gilbert pluton. These data and field relationships indicate: (1) plagioclase phenocrysts in the Mt. Gilbert pluton were not derived from the Lamarck granodiorite despite their textural similarity; but (2) the Lamarck granodiorite and its associated mafic plutons are co-intrusive as supported by the close agreement of the ages with the crystallization age obtained by Stern and others for the Lamarck granodiorite.« less
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