Coexistence Of Magmas Research Articles

Long-lived compositional heterogeneities in magma chambers, and implications for volcanic hazard

Magmas discharged during individual volcanic eruptions commonly display compositional variations interpreted as new arrivals at shallow depth of more primitive, hotter, volatile-rich magma batches mixing with resident, colder, partially degassed magma. Heterogeneities in eruption products are often interpreted as evidence of short times of order tens of hours from new magma arrival to eruption, raising concerns for emergency planning. We show here, through numerical simulations, that magma convection and mixing in a shallow magma chamber can result in long-lived, dynamically stable configurations with coexistence of magmas from nearly pure to variably mixed end-member compositions. Short mixing time scales may therefore relate to sin-eruptive processes, as heterogeneities found in the eruptive products are not necessarily the fingerprint of new magma arrival shortly preceding or triggering the eruption.

Plúton Serra da Garganta como registro de magmatismo cálcio-alcalino no Domínio Rio Piranhas – Seridó, Nordeste da Província Borborema

Existe um intenso plutonismo ediacarano, relacionado ao ciclo Brasiliano/Pan-Africano na Província Borborema, denotado por inúmeros corpos de natureza química diferente. O Plúton Serra da Garganta, situado na porção central do Domínio Rio Piranhas–Seridó, extremo NE desta Província, foi alvo de estudos cartográfico, petrográfico, de química mineral, litoquímico e geocronológico (U-Pb, zircão), com o objetivo de definir a filiação magmática e a idade de cristalização. O PSG é individualizado em duas fácies: (i) biotita granodiorito a tonalito cinza claro, equigranular, fino a médio; contém K-feldspato, plagioclásio (oligoclásio - An25-23%), quartzo e biotita (máfico principal), tendo ainda anfibólio, titanita, epídoto e allanita como acessórios; pequenos cristais de minerais opacos, zircão e apatita estão presentes; e (ii) anfibólio biotita quartzo diorito cinza escuro, equigranular, fino, constituído por plagioclásio, quartzo e K-feldspato; biotita é o principal máfico seguido por anfibólio e titanita, tendo epídoto, minerais opacos, allanita, zircão e apatita como acessórios. É comum observar feições de campo indicando coexistência de magmas. Dados litoquímicos para ambas as fácies evidenciam natureza transicional entre metaluminosa e peraluminosa, com afinidade de rochas cálcio-alcalinas, o que não é comum para corpos plutônicos no extremo NE da Província Borborema. A datação U-Pb, para a fácies principal, de 598±5 Ma (MSWD = 0,37) obtida em zircões (Shrimp) confirma idade ediacarana para sua cristalização.

Volcanic systems in the San Pedro-Ceboruco graben(Nayarit, Mexico) in the light of new K-Ar geochronological data.

Six new K-Ar ages were determined for three of four volcanic systems that constitute the San PedroCeboruco graben of western Mexico, in order to give constraints on stratigraphic relationships and problems of magma genesis in an area characterized by the coexistence of magmas with different petrologic affinities (calc-alkaline and intra-plate type). Previous age determinations are scarse for this region. Samples were dated using the unspiked K-Ar sensitivity method with a mass-fractionation correction procedure. Our new data, along with those in literature, suggest that volcanism started at ~2.5 Ma in the southwest part of the graben. Between ~2.3 Ma to ~0.1 Ma volcanic activity continued in the central part of the graben and starting from ~0.5 Ma, volcanism also extended to the northern part. This northward shifting of the volcanic activity with time occurred along NW-SE trending lines which coincide with the main regional fault systems, and could be linked to the resumption of convergence after a period of near-cessation. The major phase of the magmatic activity is younger than 0.5 Ma, but the presence of volcanic activity strictly differentiated in magmatic affinities (orogenic and intra-plate), started earlier, around 1.1 Ma and is linked to sub-slab melts flowing in the mantle wedge. The age obtained on the dome complex (San Pedro-Cerro Grande Volcanic Complex) along with literature data suggest that the caldera collapse event occurred before ~1.1 Ma and was followed by intracaldera and pericaldera activities. After ~0.6 Ma, subsidence and consequent reactivation of the eastern caldera rim, can have determined the cutting of some pericaldera domes.