Calc-alkaline plutonism in the intra-oceanic Aleutian arc, Alaska

Abstract

Some of the best developed calc-alkaline plutons in an oceanic island arc are found in the Aleutian arc, Alaska. These Tertiary plutons are similar in many ways to continental granodioritic I-type plutons, but are smaller and have isotopic ratios and Rb contents that reflect their oceanic setting. Some differences between Aleutian plutons occur, as exemplified by the higher K content and more oxidized, lower-temperature mineralogy in the Hidden Bay pluton (Adak Island) compared to the Captains Bay pluton (Unalaska Island). This variability implies regional differences in arc crust that may reflect the 20 m.y. of arc history, including thickening of the crust to a critical value, that preceded the onset of calc-alkaline magmatism. The plutons appear to have formed during the waning stages of volcanic periods, when the magmas were too cool and crystalline to erupt. Differentiation of the gabbroic to leucogranodioritic units occurred in the lower to mid-crust, followed by buoyant rise to mid-to upper-crustal magma chambers where final crystallization, and segregation of aplites occurred. Mafic dikes that intrude the plutons represent magmas that were able to fracture their way through the largely solidified pluton from below. Many gabbros and mafic diorite units in the plutons are crystal cumulates, and have no equivalents among volcanic rocks and dikes. In contrast, the predominant silicic plutonic units (58 to 63 percent SiC>2) and silicic volcanic rocks (or, in hybrid magmas, mixing end members) are similar. Whether these silicic magmas are deep-crustal differentiates of a high-AI basalt or high-temperature partial melts of older mafic arc volcanic rocks is difficult to discern because of the small chemical contrast between Aleutian magmas and country rock. Either model implies a large volume of lower-crustal mafic and ultramafic rock complementary to the observed upper-crustal plutons.

Magmatic sources and tectonic setting of Gondwana margin Ordovician magmas, northern Puna of Argentina and Chile

“…An arc or backarc-like source is consistent with La/Ta ratios mostly from 22-32. As a whole, the gross features of these units are like those of calc-alkaline plutons in the Aleutian oceanic island arc in Alaska (Kay et al, 1990) consistent with the Choschas pluton being intruded into thin crust. A difference is that Choschas units have lower Sr and Na 2 O concentrations at 59-65% SiO 2 than Aleutian plutons (2.1-2.6% Na 2 O compared to >3.5% Na 2 O), which can be attributed to contamination in a thinned continental crust.…”

“…However, attention has to be paid to the fact that volcanic and plutonic units are being compared. In particular, Kay et al (1990) showed that Aleutian plutonic units have higher K 2 O (and other incompatible element) concentrations than volcanic rocks at the same SiO 2 , and argued the plutons evolved in a closed system at the end of a magmatic cycle. The similarity of the gross features of Choschas and Aleutian plutonic units could be consistent with the Choschas pluton representing the end of the magmatic cycle associated with the Cordón de Lila complex.…”

Distinctly different parental magmas for calc-alkaline plutons and tholeiitic lavas in the central and eastern Aleutian arc

“…Magmas with high SiO 2 and H 2 O become viscous, and preferentially stall during mid-crustal degassing (e.g., Cann, 1970). It is reasonable to infer that in the central and eastern Aleutians, such magmas predominantly stall to form the calc-alkaline plutons, rather than erupting as previously proposed (e.g., Kay et al, 1990;Kay and Kay, 1994;Kelemen et al, 2003a). In the thinner, faulted crust of the western Aleutians, such magmas erupt to form many small cones on the seafloor.…”

“…In this case, the dominance of calc-alkaline compositions among the plutons could reflect higher SiO 2 , f O 2 , and/or H 2 O contents in their parental magmas, compared to the basaltic, more reduced magmas with lower H 2 O contents that are parental to Aleutian tholeiitic lavas (Kay et al, 1990;Kelemen, 2003aKelemen, , 2003bKelemen, , 2014.…”

“…Rioux et al, 2010). In the Aleutian arc, seismic data require a mafic bulk composition for the arc lower crust (Holbrook et al, 1999;Fliedner and Klemperer, 1999;Shillington et al, 2004); however, the upper and middle crust in the Aleutians contain abundant intermediate composition plutons (e.g., Kay et al, 1982Kay et al, , 1990Kay and Kay, 1994;Perfit et al, 1980;Yogodzinski et al, 1993).…”