Along-strike variation in the Aleutian Island Arc: Genesis of high Mg# andesite and implications for continental crust

Summary. Quartz-keratophyres from an ensimatic island arc on Bømlo, southwest Norwegian Caledonides, resting on the Lykling ophiolite, have yielded a Rb/Sr whole rock age of 535 ± 46 Ma. This Cambrian age, which is a minimum age of formation for the ophiolite, indicates an early phase of subduction, corresponding with the orogenic activity known from northern Norway, i.e. the Finnmarkian. Before Middle Ordovician time, the ophiolite and ensimatic arc had been obducted onto continental crust and deformed, as they are unconformably overlain by a volcanic complex of subaerial rhyolites and andesites which have yielded Rb/Sr whole rock ages of 464 ± 16 and 468 ± 23 Ma respectively.

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Timing of events in an Early Cretaceous island arc–marginal basin system on South Georgia

Geological Magazine ◽

10.1017/s0016756800043582 ◽

1979 ◽

Vol 116 (3) ◽

pp. 167-179 ◽

Cited By ~ 20

Author(s):

P. W. G. Tanner ◽

D. C. Rex

Keyword(s):

Continental Crust ◽

Island Arc ◽

Magmatic Arc ◽

South Georgia ◽

Marginal Basin ◽

Early Mesozoic ◽

Mafic Complex ◽

Minimum Age ◽

Radiometric Ages ◽

Basin System

Summary19 new K–Ar mineral ages of 78-201 Ma and 3 Rb–Sr whole rock isochron ages of 81 ± 10, 127±4 and 181±30 Ma are presented from units of continental crust, mafic complex and island arc assemblage on South Georgia. The Drygalski Fjord Complex, part of the possible floor of the marginal basin in the southern part of the island, includes granodiorite and gabbro plutons of minimum age 180–200 Ma. Together with older metasediments they have been affected by a major thermal event at about 140 Ma, thought to have resulted from the emplacement of a mafic complex (Larsen Harbour Formation) during the initial opening of the marginal basin. Rocks of the Larsen Harbour Formation are cut by the Smaaland Cove intrusion dated by Rb–Sr whole rock isochron at 127±4 Ma. An island arc assemblage exposed to the SW of South Georgia consists of pyroclastic rocks cut by monzodiorite and andesite intrusions, which give radiometric ages of 81–103 Ma. These data suggest that the marginal basin opened during the late Jurassic (pre-140 Ma); that part of an earlier (early Mesozoic) magmatic arc is preserved in continental crust making up part of the floor of the basin; and that subduction continued beneath the island arc until at least the Senonian time. The younger plutons in the arc were emplaced at roughly the same time as turbidite facies rocks at deep levels in the marginal basin were being affected by penetrative deformation and metamorphism. The timing of events on South Georgia agrees closely with that deduced for the continuation of the same island arc–marginal basin system in South America. The 180–200 Ma plutons correlate with an older suite of plutonic rocks reported from the Antarctic Peninsula and southern Andes; they are part of a once-continuous magmatic arc related to subduction of the Pacific plate beneath Gondwanaland during the early Mesozoic.

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A Late Mesozoic island arc in the southern Andes, Chile

Geological Magazine ◽

10.1017/s0016756800043594 ◽

1979 ◽

Vol 116 (3) ◽

pp. 181-190 ◽

Cited By ~ 22

Author(s):

M. Suárez

Keyword(s):

Continental Crust ◽

Rift Zone ◽

Volcanic Rocks ◽

Upper Jurassic ◽

Island Arc ◽

Volcanic Complex ◽

Late Mesozoic ◽

Marginal Basin ◽

The Pacific ◽

Pacific Side

SummaryThe Hardy Formation, a sequence of Upper Mesozoic volcanic rocks exposed in Peninsula Hardy (Isla Hoste) in the southernmost archipelago of Chile represents, at least in part, the island-arc assemblage of an island-arc-marginal-basin system related to an eastward dipping subduction zone. This island arc was founded on South American continental crust and is also represented in the island of South Georgia 2000 km to the E. The island-arc assemblage includes pyroclastic rocks, characterized by a high proportion of vitric material, and lava intercalations ranging in composition from rhyolite to basalt. These rocks underwent zeolite and prehnite-pumpellyite facies metamorphism and are gently folded, in contrast with the intense folding exhibited by the rocks exposed to the north of Peninsula Hardy. Silicic volcanics assigned to this assemblage underlie pillow lavas, and are intruded by dolerites and gabbros probably related to a Late Jurassic-Early Cretaceous ophiolite magmatism associated with the generation of a quasioceanic marginal basin. Volcanic turbidites (Yahgan Formation) were deposited into the marginal basin.It is suggested that in pre-marginal basin times the Hardy Formation interfingered towards the Atlantic with the silicic volcanics of the Tobifera Formation. However, recent geochemical work on the Tobifera Formation suggest an origin by continental crust anatexis in a volcano-tectonic rift zone related to upper mantle diapirism, whereas an island arc origin is favoured for at least the andesitic and basaltic components of the Hardy Formation. Therefore, the geology of Peninsula Hardy as presented here, confirms early assumptions of the splitting apart of a Middle–Upper Jurassic volcanic terrain along the Pacific margin of South America during the generation of a marginal basin. The spreading axis of the latter seems to have been located at the boundary of two somewhat overlapping petrotectonic assemblages: and island arc on the Pacific side and a silicic volcano-tectonic rift zone towards the Atlantic. A probably Cenozoic volcanic complex discordantly overlies the Yahgan and Hardy formations.

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