Capo Marargiu Volcanic District (CMVD) is an Oligo-Miocene calc-alkaline complex located in northwestern Sardinia (Italy) and characterized by the widespread occurrence of basaltic to andesitic domes. One of these domes hosts abundant crystal-rich enclaves with millimeter-to-centimeter sized clinopyroxenes showing intriguing textural features as a result of complex magma dynamics. To better understand the mechanisms governing the early evolution of the CMVD magmatic system, such clinopyroxene phenocrysts have been investigated in terms of their major, trace element, and isotopic compositions. Three distinct clinopyroxene populations have been identified, i.e., Type 1, Type 2, and Type 3. Type 1 appears as the sub-rounded cores of diopsidic clinopyroxenes with overgrowth textures corresponding to Type 2 and Type 3. These latter populations may also occur as single isolated crystals. Type 2 diopsidic pyroxene exhibits oscillatory zoning and spongy cellular textures with Type 3 overgrowths, whereas Type 3 are polycrystalline augitic glomerocrysts with occasional Type 2 overgrowths. The crystal overgrowths are striking evidence of magma recharge dynamics. Type 1 (CpxMg#83–92), Type 2 (CpxMg#75–82), and Type 3 (CpxMg#72–79) are, respectively, in equilibrium with Sardinian mantle-derived high-Mg basalts (HMB with meltMg#56–73), least differentiated basaltic andesites (BA with meltMg#45–56) and evolved basaltic andesites (EBA with meltMg#41–50). Type 1 and Type 2 are diopsidic phenocrysts that have evolved along a similar geochemical path (i.e., linear increase of Al, Ti, La, and Hf contents, as well as negligible Eu-anomaly) controlled by olivine + clinopyroxene + amphibole fractionation. This differentiation path is related to phenocryst crystallization from hydrous HMB and BA magmas stalling at moderate crustal pressures. The occurrence of globular sulfides within Type 1 suggests saturation of the HMB magma with a sulfide liquid under relatively low redox conditions. Moreover, Type 1 clinopyroxenes show variable 87Sr/86Sr ratios ascribable either to assimilation of crustal material by HMB magma or a mantle source variably contaminated by crustal components. In contrast, Type 3 augitic phenocrysts recorded the effect of plagioclase and titanomagnetite fractionation (i.e., low Al and Ti contents associated with high La and Hf concentrations, as well as important Eu-anomaly) from more degassed EBA magmas ponding at shallow depths. Rare titanite associated to Type 3 and titanomagnetite crystals point to high oxidizing conditions for EBA magmas. The 87Sr/86Sr ratios of both Type 2 and Type 3 are almost constant, suggesting a limited interaction of BA and EBA magmas with the country rock. The overall textural and compositional features of Type 1, Type 2, and Type 3 clinopyroxene phenocrysts lead to the conclusion that CMVD was characterized by a polybaric plumbing system where geochemically distinct magmas crystallized and mixed under variable environmental conditions.
Accessory mineral U–Pb geochronology by isotope dilution thermal ionization mass spectrometry (ID-TIMS) requires precise and accurate determinations of parent–daughter isotope ratios.
The Karavansalija Mineralized Center (KMC) with its Au–Cu skarn mineralization associated with the Rogozna Mountains magmatic suite in southwestern Serbia belongs to the Oligocene Serbo-Macedonian magmatic and metallogenic belt (SMM-MB). Samples from intrusive and volcanic rocks at the KMC show typical arc signatures of subduction-derived magmas through enrichment in large-ion lithophile elements (LILE) and depletion of high–field strength elements (HFSE). The magmas developed a high-K (calc-alkaline) fractionation trend and evolved toward shoshonitic compositions. Whole-rock trace element data suggest plagioclase-absent, high-pressure amphibole ± garnet fractionation that generates adakite-like hydrous magmas during evolution in lower crustal magma chambers. Zircon LA–ICP–MS and high-precision CA–ID–TIMS dating together with zircon trace elements and Hf isotope measurements were carried out in order to couple the geochronologic and geochemical evolution of the KMC. The results suggest that magmatism starts around 29.34 Ma with granitic to rhyodacitic subvolcanic intrusions followed by a more evolved magmatic intrusion that was emplaced into Cretaceous limestone, generating a widespread skarn alteration at ca. 28.96 Ma. After a period of quiescence of about 1.2 My, either another magma body evolved or the same upper crustal magma chamber was recharged and also likely partly reactivated older plutonic rocks as indicated by xenocrysts. The REE ratios shift from apatite, titanite ± amphibole-dominated fractionation of the older magmatic event to crystallization of allanite, efficiently depleting the LREE and Th/U in the younger upper crustal magma. After a lamproite-like melt was injected, the increased heat and fluid pressure led to the expulsion of a quartz-monzonite porphyritic stock at ca. 27.72 Ma, strongly interacting with the skarns and established a fertile hydrothermal system. Soon after a non-mineralized second pulse of some porphyry dykes cut the previous phenocryst-rich “crowded” porphyries and skarns at ca. 27.60 Ma, thus bracketing the maximum timespan of ore mineralization to about 112 ± 45 Ka. Increased contribution of a lamproite-like melt is inferred from the presence of phlogopite micro-phenocrysts, phlogopitization of biotite, and diopside clusters in the latest porphyry dykes. There is a trend of increased crustal assimilation from the oldest volcanic phase to the emplacement of the youngest porphyry dykes recorded by ɛ-Hf of the zircons. Oligocene occurrences of significant base metal mineralization within Serbia, northern Macedonia, and Greece, e.g., Crnac, Rudnik, Veliki Majdan, Stratoniu, or the Cu–Au porphyry at Buchim (northern Macedonia), are all associated with trachy-andesitic (quartz latitic) porphyry dykes, which originated through post-collisional tectonic settings or upper plate extension involving reworking of crustal arc-derived rocks and partial melting of the mantle wedge. This study demonstrates that on the basis of field relationships and the application of high-precision CA-ID-TIMS zircon age data, pulses of porphyry dykes of a 10ka age range can be distinguished, and the timing of mineralization can be parenthized.
Integration of several geologic lines of evidence reveals the prevalence of a lowland trans-Andean portal communicating western Amazonia and the westernmost Andes from at least middle Miocene until Pliocene times. Volcanism and crustal shortening built up relief in the southernmost Central and Eastern Cordilleras of Colombia, closing this lowland gap. Independent lines of evidence consist first, of field mapping in the Tatacoa Desert with a coverage area of ∼381 km 2 , 1,165 km of geological contact traces, 164 structural data points, and 3D aerial digital mapping models. This map documents the beginning of southward propagation of the southernmost tip of the Eastern Cordillera’s west-verging, fold-and-thrust belt between ∼12.2 and 13.7 Ma. Second, a compilation of new and published detrital zircon geochronology in middle Miocene strata of the Tatacoa Desert shows three distinctive age populations: middle Miocene, middle Eocene, and Jurassic; the first two sourced west of the Central Cordillera, the latter in the Magdalena Valley. Similar populations with the three distinctive peaks have now been recovered in western Amazonian middle Miocene strata. These observations, along with published molecular and fossil fish data, suggest that by Serravallian times (∼13 Ma), the Northern Andes were separated from the Central Andes at ∼3°N by a fluvial system that flowed into the Amazon Basin through the Tatacoa Desert. This paleogeographic configuration would be similar to a Western Andean, or Marañon Portal. Late Miocene flattening of the subducting Nazca slab caused the eastward migration of the Miocene volcanic arc, so that starting at ∼4 Ma, large composite volcanoes were built up along the axis of today's Central Cordillera, closing this lowland Andean portal and altering the drainage patterns to resemble a modern configuration.
