Do extrusion ages reflect magma generation processes at depth? An example from the Neogene Volcanic Province of SE Spain

The high-K calc-alkaline volcanic rocks along the Neogene Volcanic Province of SE Spain represent crustal anatectic melts mixed with mantle components during the opening of the Alboran Sea. Partially melted metapelitic enclaves, along with the geochemical signature, provide evidence of their crustal source. U-Pb SHRIMP geochronology on monazite and zircon from enclaves and their hosting lavas in the localities of El Hoyazo, MazarrA(3)n and Mar Menor reveals variable delays between the melting at depth and the eruption of the volcanics. These data indicate that: (1) the most important event of anatexis in the Neogene spanned at least the 3 m.y. interval between 12 and 9 Ma; (2) there is no trend in age of crustal melting; and (3) the delay between magma generation and extrusion varies from more than 3 m.y. at El Hoyazo to similar to 0.5 m.y. and possibly 2.5 m.y. at Mar Menor, with no significant delay measurable at MazarrA(3)n. The variable time delay between anatexis and lava extrusion indicates that radiometric ages of volcanics may provide misleading information on the timing of magma genesis occurring at depth. This highlights the pitfall of basing detailed geodynamic models on volcanic extrusion ages alone

Armouring effect on Sr-Nd isotopes during disequilibrium crustal melting: the case study of frozen migmatites from El Hoyazo and Mazarron, SE Spain

Crustal melting is responsible for the production of large volumes of rhyolitic melt and therefore is central to understand the rheology of the crust and the mechanisms of crustal differentiation. The attainment of isotopic equilibrium during melting of crustal rocks is implicitly assumed in most isotopic dating and tracing studies. This assumption considers the melting event as an instantaneous process and does not take into account the duration of anatexis. To assess the critical role of the timescale of crustal melting, we have studied the unique occurrence of erupted migmatites enclosed as xenoliths in the El Hoyazo and Mazarron dacites of the Neogene Volcanic Province of SE Spain. These xenoliths represent the residue after some 30-60 % rhyolitic melt extraction at P-T conditions of 5-7 kbar and similar to 850 degrees C, and consist of biotite, plagioclase, sillimanite, garnet, cordierite, graphite and abundant glass inclusions (i.e., not extracted rhyolitic melt) within each mineral phase. The timescale of melt extraction was similar to 3 Myr and < 0.8 Myr at El Hoyazo and Mazarron, respectively, resembling the duration of melting events during rapid anatexis caused by basalt underplating and crustal assimilation processes. In both localities, the minerals and glass inclusions of erupted migmatites preserve a significant Sr and minor Nd isotope disequilibrium. At Mazarron the isotopic disequilibrium is most marked owing to the shorter residence time of the melt within the source. The isotopic disequilibrium is not caused by the major xenolith-forming minerals but rather by the accessory phosphate inclusions (apatite +/- monazite +/- xenotime) hosted in garnet and biotite. The preservation of isotopic disequilibrium in these accessory phases has been facilitated by both their intrinsically low Sr and Nd diffusion coefficients and the armouring effect caused by their occurrence within biotite and garnet crystals, which acted as chemical barriers to Sr and Nd diffusion. This result implies that modelling of radiogenic isotope equilibration in natural systems should consider elemental diffusion in a composite medium with a resistance at the interface, i.e. different partition coefficients between adjacent mineral phases

What drives the distribution in nature of 3T vs. 2M(1) polytype in muscovites and phengites? A general assessment based on new data from metamorphic and igneous granitoid rocks

Petrologic, chemical, and polytype data are presented for dioctahedral potassic micas from K-feldspar-bearing metamorphic and igneous rocks of acidic composition unaffected by high-pressure (HP) conditions. The paper aims to demonstrate that: (1) under non-HP conditions, in both metamorphic and igneous plutonic environments, a given bulk-rock compositional constraint imposes a more or less marked phengitic composition to dioctahedral potassic mica; and (2) this muscovite crystallizes as 2M1, notwithstanding its phengitic composition. The samples (157 in number) are from widespread provenances. We conclude that the growth of 3T polytype of muscovite is not a function of mica composition. This is consistent with the recent crystallographic knowledge on polytypism, cation ordering, elastic properties, and structural deformational mechanisms of muscovite, which address the stabilization of 3T with pressure. Keywords: Muscovite, phengite, celadonitic substitution, polytypism, 2M1, 3T, pressure-polytype relationship, mica chemistry, polytype relationship, petrologic mineralog