Timescales of plutonic-subvolcanic-volcanic connection in a Mio-Pliocene long-lived igneous system (Tuscany): zircon CA-ID-TIMS dating

<p>The genetic link between plutonic and volcanic realms is a key for understanding timescales of igneous plumbing systems, and precise geochronological records are pivotal in estimating the duration of processes at different levels in such plumbing systems. The Campiglia igneous complex, Tuscany, offers exposures of the full range of emplacement levels (plutonic, subvolcanic, volcanic) of mantle- and crust-derived magmas. Magma emplacement occurred astride the Miocene-Pliocene boundary. New high-precision U-Pb CA-ID-TIMS, zircon geochronological data, coupled with LA-HR-ICP-MS zircon dates for the whole Campiglia system define a short crystallization time span for zircon from the peraluminous granite pluton (~100 ka), intermediate for the shallow-level mafic porphyry (~450 ka), and longer for the rhyolite (~700 ka), at odd with what commonly expected. The oldest ages for the three units are the same, leading to hypothesize the occurrence of a bimodal deep reservoir remaining in melt-present conditions for some 700 ka. In this framework, early-crystallized zircons were cannibalized by younger melt batches that were sequentially extracted from the reservoir.</p>

Petrological Features of Korsun'-Novomyrhorod Anorthosite-Rapakivi Granite Pluton

The Korsun’-Novomyrhorod pluton is the second after the Korosten one in terms of the scale of Proterozoic (1757-1748 Ma) anorthosite-rapakivi-granite magmatism in the Ukrainian Shield. According to geochronological data, pluton was formed as a result of multiple ascending and crystallization of basic to acidic melts. Differentiation of initial melts because to be responsible for gabbro-anorthosite and monzonites series crystallization. Whereas rapakivi granites, which are predominate in the modern erosion level, were formed from felsic magma not directly related with differentiation of basic melt. In view of the current level of mineralogical research, it is difficult to use modern geobarometry methods to reliably estimate the depth of rocks crystallization. At the same time, a number of factors (absence of volcanic and dike analogues of basic rocks, insignificant distribution of pegmatite bodies, predominance of high-Fe mafic minerals, absence of primary magnetite, etc.) indicate deeper conditions for rocks disclosed by modern erosional cut in comparition to similar Korosten pluton. Therefore, the liquid line of dissent, petrological and mineralogical features of the rocks can be explained by the reducing (low fO2) or abyssal conditions of their formation. It is possible that the deeper conditions of crystallization of parental melt are due to more distinctly developed syenitic trend of evolution with the appearance of high-Fe syenites during final stages. Preliminary data indicate on possibility of vertical layering of gabbro-anorthosite massifs, which manifested by increasing proportion of high-Fe basic rocks with depth. Available isotope-geochemical studies do not provide unambiguous data on regarding reservoirs of primary melts implaying both mantle and mixed mantle-crustal their origin. The evolution of the petrochemical features of basic rocks, in our opinion, is in better agreement with their formation as result of differentiation of the primary high-alumina tholeiitic melt, significantly contaminated by lower crustal material. This determined the subalkaline nature of basic rocks and a significant predominance of norites, in comparition to more typical gabbros, and monzonites. In contrast to the previously proposed hypotheses of the formation of intermediate rocks because of partial melting of felsic rocks by basic intrusions, or mingling of basic and acidic melts, some of petrochemical features and geological position can be satisfactorily explained by their crystallization from the residual melt.

Dyke Rocks of the Korsun-Novomyrhorod Anorthosite-Rapakivigranite Pluton

In spite of significant intensity and composition variability of dyke magmatism in the Ingul mega-block, in the Korsun- Novomyrhorod anorthosite-rapakivi granite pluton (KNP) dyke magmatism is restricted. To present day, dykes of basic composition have not been described in this pluton. Previously mentioned gabbro-diabases dykes or diabase porphyrites should be classified as medium rocks as well as they are enriched by SiO2, Na2O and K2O. The authors succeeded studied the dyke from the Nosachiv massif, which can be considered the only known dyke of basic composition in KNP. By chemical composition, this rock is similar to the Ti-enriched gabroids of KNP, but differs in the presence of high-Ti magnetite (not typical mineral in plutonic rocks of KNP), and in terms of SiO2 and Na2O + K2O it should be classified as subalkaline. Most of the dikes known in the KNP are mainly monzonites, monzosienites, less widespread syenites, granite dikes are quite rare too. As our studies have shown, in the previously described porphyry dykes diabase structure is not exihibited. Furthermore pyroxene content is the first percent, and a significant (or most) of them are probably xenogeneic. According to chemical composition such dykes should be classified as medium in composition, because the SiO2 content in them is in the range of 54-60 % and elevated content of K2O, often prevails over Na2O, by low MgO and higher TiO2 they are similar to gabroid of KNP. In these rocks, normative orthoclase and quartz are calculated in significant quantities. Therefore, we offer to consider these rocks as dyke analogues of monzonites (quartz-containing or quartz) and regard as monzonite porphyry. The significant distribution of monzonites and syenites and their dyke analogues denote clear pronounced monzonite-syenite trend of evolution in KNP.

Geochemistry of trace elements in rock-forming minerals of gneisses and granites of the Murzinka granite area, Central Urals

The Murzinka granite area (Central Urals), which combines Murzinka granite pluton and underlying rocks of the Murzinka-Adui metamorphic complex, exhibits an evident wetrending geochemical zonation of magmatism with increasing of Rb, Li, Nb and Ta contents and decreasing ba and Sr contents and K/Rb, zr/Hf and Nb/Ta ratios from vein granites of the Yuzhakovo complex to granites of the Vatikha complex and further to granites of the Murzinka complex (Fershtater et al., 2019). To develop the ideas about geochemical zonation of the Murzinka granite magmatism, as well as about the role of gneisses of the Murzinka-Adui metamorphic complex in the formation of granites, we studied the distribution of trace elements in biotite and feldspars of gneisses and granites. Biotite shows an increase in Li, Rb, Cs, Nb, Ga, zn, Mn, Sc, Sn and Tl contents and a decrease in V, Cr, Co, Ni, Y, zr and ba contents from vein biotites of the Yuzhakovo granites to two-mica granites of the Murzinka complex. The composition of feldspars also changes in this direction: plagioclase is enriched in Li, Rb, Cs, be, zn and depleted in Sr, ba, Ga and Pb and K-feldspar is enriched in Rb and depleted in Sr and ba. The varying trace element composition of rock-forming minerals of gneisses and granites is explained by We-trending change in the composition of a crustal protolith, as well as the formation conditions of granites. Figures 6. Tables 4. References 17.