Injection Of Mafic Magma Research Articles

Complex interaction of aluminous metasedimentary xenoliths and siliceous magma; an example from Mt. Amiata (Central Italy)

Aluminous, silica-deficient metasedimentary xenoliths in siliceous lavas of Mt. Amiata have preserved composite zoning-patterns indicative for complex processes of magma-rock interaction. Petrographic observations and small-scale mineralogical and chemical differences between up to five distinct zones (including the core and envelope of lava) provide evidence that: Quartz-poor muscovite-biotite schists are considered to be the most likely parent rocks and it is believed that release of volatiles from decomposing micas played a significant role in the high-temperature metamorphic evolution of the xenoliths and their interaction with the magma. The conditions favourable for assimilation were enhanced by injection of mafic magma into the magma chamber. Although this concurrent operation of magma-mixing precludes a quantitative estimate of contamination from the wall-rocks (which was probably of minor importance) the present example indicates that dry acid magma may potentially become more mafic by interaction with partially melted hydrous rocks.

Minette inclusions in the rhyodacitic lavas of Mt. Amiata (Central Italy): Mineralogical and chemical evidence of mixing between Tuscan and Roman type magmas

Abstract Major-element, trace-element and mineralogical data are reported for a series of mafic inclusions occurring in the rhyodacitic lavas of Mt. Amiata (southern Tuscany, Italy), a volcanic complex belonging to the Tuscan Magmatic Province. The inclusions form, together with some subordinate mafic latitic lavas and the host rock rhyodacites, a continuous series, which varies from olivine and minor nepheline normative to quartz, hypersthene and corundum normative rock-types over a range of 48–67% SiO 2 . All of the rocks are characterized by high contents of potassium (5–6% K 2 O) and many trace elements, particularly the incompatible elements. Following straight-line correlation trends, Al, Fe, Mg, Ca, Ti, P, Sr and Ba decrease with increasing SiO 2 , whereas Na and Li increase, and K, Rb, Zr, La, Ce, Nb and Y remain virtually constant. The primary mineral assemblage of the inclusions consists of diopsidic pyroxene, forsteritic olivine and mica, and is representative for minettes. Xenocrysts of plagioclase, sanidine, orthopyroxene clinopyroxene, biotite, ilmenite and olivine occur in increasing amounts in the more siliceous samples. The composition of these xenocrysts is identical to that of the phenocryst assemblage in the rhyodacites. The latitic lavas are rich in xenocrysts and differ from the minettes mainly by the absence of primary mica. The data strongly suggest a magma mixing origin for the minettes, latites and possibly part of the rhyodacites. Injection of mafic magma into a siliceous magma chamber has resulted in the formation of hybrid inclusions and a latitic magma that could only reach the surface after eruption of the voluminous rhyodacitic lavas. The rhyodacites which erupted early and are slightly richer in SiO 2 than most of the other rhyodacites are considered to represent the acid mixing component, whereas the mafic end-member has close affinities to the potassic alkaline lavas of the adjacent Roman Province. The inclusions have the modal composition of a minette and this implies that volatiles were important constituents of the hybrid magmas. This may be attributed to the primary character of the Roman-type end-member involved, or explained by a process of selective enrichment operating during mixing. Although upper mantle derived Roman magmas have interacted with Tuscan magmas of crustal anatectic origin at Mt. Amiata and probably other volcanic centres in central Italy, the specific chemistry of lavas in both provinces cannot be explained by simple mixing of these magmas in shallow chambers.

The response of a stratified ocean to stationary or moving wind-systems : Longuet-Higgins M.S., 1965. Deep-Sea Res., 12 (6): 923–973.

Compositional features of 93 samples of primitive Pliocene to recent basalts erupted along the Brothers Fault Zone in the northernmost Basin and Range indicate that they were derived from a shallow mantle source and underwent only minor shallow-level fractionation. Simple mass-balance modelling can derive these basaltic bulk compositions by removal of small amounts of observed crystalline phases from glass compositions produced in peridotite melting experiments. Additional support comes from phase equilibria data on other magnesian basalts having similar bulk compositions. The eruption of these lavas without substantial subcrustal fractionation was probably promoted by progressive extension along the Brothers Fault Zone. This origin is in sharp contrast to that generally proposed for mid-Miocene Columbia River and Steens Mountain basalts, which show clear evidence in their evolved compositions (e.g. Mg # ~ 40) of having stagnated at shallow depth where they differentiated to nearly basaltic andesite compositions. Bulk compositions of northern Basin and Range silicic rocks, together with physical and thermal considerations, suggest that they, like their counterparts in the Snake River Plain, were products of crustal anatexis driven by the injection of mafic magmas, but with meta-volcaniclastic protoliths rather than Archaean basement rocks, as in the case of the Snake River Plain rhyolites. These petrologic features suggest that the arrival of the mantle plume presently beneath Yellowstone produced or strongly influenced most late Cenozoic magmatism in the Oregon northern Basin and Range. This model accounts for many features of the northern Basin and Range in Oregon: (1) the change in basaltic character about 10 to 8 Ma ago from voluminous, evolved Columbia River/Steens lavas to smaller-volume primitive lavas and the lack of younger lavas atop the Columbia River Basalts; (2) the lack of an obvious track of the Yellowstone hot spot west of the Oregon-Idaho-Nevada tri-state area; (3) the “mirror-image” age relationship of silicic rocks in the northern Basin and Range and Snake River Plain; (4) the formation of silicic rocks by crustal anatexis and the general decrease in their volumes with time in Oregon but not along the Snake River Plain; (5) the high elevation of the region; and (6) the high surface heat flow in the Oregon northern Basin and Range. The proposed model obviates the controversy surrounding the pre-Miocene history of the Yellowstone plume by proposing that the plume initiated about 18 Ma ago.