Suzanne Mahlburg Kay

Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge

The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip–dominated western Aleutian arc. Two types of chemically and petrologically distinctive primitive andesites have been found among the Miocene–late Pleistocene–age volcanic rocks in the western Aleutians. These are termed the “Adak-type” and “Piip-type” magnesian andesites. Trace element and isotopic characteristics indicate that Adak-type magnesian andesites (adakites) formed principally as small percentage melts of the basaltic portion of the subducting oceanic crust, leaving a clinopyroxene-garnet-rutile residual mineralogy. The resulting slab melt signature (high La/Yb, Sr) distinguishes Adak-type magnesian andesites from all other Aleutian volcanic rocks. Primitive characteristics (high Mg#, Cr, Ni) and intermediate compositions (∼59% SiO2) of Adak-type magnesian andesites were acquired by interaction with peridotite and/or basalt in the mantle wedge. The absence of olivine phenocrysts from Adak-type magnesian andesites indicates that they were not equilibrated with peridotite and so are unlike Piip-type magnesian andesites, which appear to have equilibrated under low pressure and hydrous conditions in the subarc mantle. Piip-type magnesian andesites also contain a slab melt component, but reaction-equilibration with peridotite has lowered La/Yb and Sr to levels like those of common Aleutian volcanic rocks. Miocene-age calc-alkaline rocks of the Komandorsky Islands have chemical characteristics transitional between those of Adak-type magnesian andesites and common Aleutian volcanic rocks from the central and eastern arc. In a source mixture of depleted mantle wedge, slab melt, and sediment, the Komandorsky rocks have a relatively large contribution from the slab melt endmember. The strong slab melt signature among western Aleutian rocks is attributed to highly oblique convergence that produced a slow subduction path into the subarc mantle. Geochemically, the slab melt provided a high Sr, La/Yb, La/Ta, and low Ti/Hf endmember to the western Aleutian source mixture. The enhanced role for slab melting in the western Aleutians may be like that predicted for Archean systems and for modern systems where the subduction zone is warm. In this regard, Adak-type magnesian andesites are probably the appropriate analog to sanukitoids and other primitive andesitic rocks of Archean age.

Episodic arc migration, crustal thickening, subduction erosion, and magmatism in the south-central Andes

The past ~25 m.y. of geologic history in the northern ~300 km (~33°–36°S) of the Andean Southern Volcanic Zone has seen waxing and waning magmatic production rates and episodic eastward relocation of arc segments accompanied by abrupt chemical changes in the magmas. These changes can be linked to episodes of crustal thickening at times of backarc thrusting and to peaks of subduction erosion of forearc crust and mantle lithosphere at times of frontal-arc migration to the east. The magmatic-tectonic coupling is well seen in the history—enhanced by 28 new K-Ar ages, >160 major and trace element analyses, and Sr, Nd, and Pb isotope analyses—of a west to east transect through the El Teniente copper district near 34°S. The temporal trends in magmatic chemistry in this transect are like the well-documented south to north trends in Pleistocene to Holocene volcanic centers of the Southern Volcanic Zone, and both can be linked to the same events. The magmatic changes require differences in magma source regions as shown by isotopic data and in depths of crustal magma generation/fractionation as shown by pressure-sensitive trace element distributions. “Adakitic” magmas in the region are attributed to a combination of melting the base of thickened lower crust and crust entering the mantle through subduction erosion. Subduction erosion is argued to peak in episodes of frontal-arc migration at ca. 19–16 Ma and ca. 7–4 Ma. The combined effects of crustal shortening and forearc truncation in the past 20 m.y. near 34°S have led to the loss of ~170 km of crustal width. The timing and arc length over which these events occurred show that subduction of the Juan Fernandez Ridge on the Nazca plate cannot have been the major driving force. The history of the region shows the importance of non-steadystate processes in arc-magma production and the necessity of studying arc systems over millions, not tens of thousands, of years.

Young mafic back arc volcanic rocks as indicators of continental lithospheric delamination beneath the Argentine Puna Plateau, central Andes

The spatial distribution of some major and trace element and isotopic characteristics of backarc Plio-Quaternary basaltic to high-Mg andesitic (51% to 58% SiO2) lavas in the southern Puna (24°S to 27°S) of the Central Andean Volcanic Zone (CVZ) reflect varying continental lithospheric thickness and the thermal state of the underlying mantle wedge and subducting plate. These lavas erupted from small cones and fissures associated with faults related to a change in the regional stress system in the southern Puna at ≈ 2 to 3 Ma. Three geochemical groups are recognized: (1) a relatively high volume intraplate group (high K; La/Ta ratio 25) that occurs over intermediate thickness lithosphere on the margins of the seismic gap and behind the main CVZ and represents an intermediate percentage of mantle partial melt, and (3) a small-volume shoshonitic group (very high K) that occurs over relatively thick continental lithosphere in the northeast Puna and Altiplano and represents a very small percentage of mantle partial melt. Mantle-generated characteristics of these lavas are partially overprinted by mixing with melts of the overlying thickened crust as shown by the presence of quartz and feldspar xenocrysts, negative Eu anomalies (Eu/Eu 0.7055) and Pb and nonradiogenic Nd ( eNd < −0.4) isotopic ratios. Mixing calculations show that the lavas generally contain more than 20% to 25% crustal melt. The eruption of the intraplate group mafic lavas, the change in regional stress orientation, and the high elevation of the southern Puna are suggested to be the result of the late Pliocene mechanical delamination of a block (or blocks) of continental lithosphere (mantle and possibly lowermost crust). The loss of this lithosphere resulted in an influx of asthenosphere that caused heating of the subducting slab and yielded intraplate basic magmas that produced extensive melting at the base of the thickened crust. Heating of the subducting slab led to formation of the seismic gap and trenchward depletion of the slab component. Backarc calc-alkaline group lavas erupted on the margins of this delaminated block, whereas shoshonitic group lavas erupted over a zone of relatively thick nondelaminated lithosphere to the north.

Late Paleozoic to Jurassic silicic magmatism at the Gondwana margin: Analogy to the Middle Proterozoic in North America?

A vast region of upper Paleozoic to Middle Jurassic (300-150 Ma) silicic magmatic rocks that erupted inboard of the Gondwana margin is a possible Phanerozoic analogue to the extensive Middle Proterozoic (1500-1350 Ma) silicic magmatic province that underlies much of the southern mid-continent of North America. Like the North American rocks, the Gondwana silicic magmas appear to be melts of crust that formed about 200-300 m.y. earlier. In the North American case, this older crust formed and was accreted to the continent during a major period of crustal formation (1700-1900 Ma), whereas in the Gondwana case, the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the Paleozoic. In both cases, basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older (and previously melted) crust. The similarities between the North American and Gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas, ponded at the base of the preheated crust, caused extensive crustal melting that inhibited upward passage of the basalts. In both cases, silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes, and before breakup of the supercontinent. By analogy with the polar wander curves for Gondwana, the granite-rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles.

Southern Patagonian plateau basalts and deformation: Backarc testimony of ridge collisions

Abstract The distribution and volume of Tertiary Patagonian plateau basalts and the evolution of the Patagonian fold and thrust belt between 46° and 49°S appear to be closely tied with ridge-trench interactions along the continental margin to the west. Eocene plateau basalts south of 43°S are associated with a gap in the Eocene arc and can be related to a proposed collision of the Aluk-Farallon ridge against the trench at this time. Eruptions of Late Miocene to Recent plateau basalts between 46° and 49°S can be related to time-transgressive slab windows in the underlying mantle generated by the collision of segments of the Chile ridge at 10–14 Ma and 6 Ma. The principal deformation in the Patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with Late Miocene ridge collision. Comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment. Almost all Eocene to Recent plateau basalts between 46° and 49°S have OIB-like (ocean island basalt) trace element signatures showing that the mantle source region changed little with time. Plateau basalts with more arc-like signatures appear to be contaminated by continental crust. Relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows.

Central Andean Ore Deposits Linked to Evolving Shallow Subduction Systems and Thickening Crust

Major Miocene central Andean (lat 22°–34°S) ore districts share common tectonic and magmatic features that point to a model for their formation over a shallowing subduction zone or during the initial steepening of a formerly flat subduction zone. A key ingredient for magmatism and ore formation is release of fluids linked to hydration of the mantle and lower crust above a progressively shallower and cooler subducting oceanic slab. Another is stress from South American–Nazca plate convergence that results in crustal thickening and shortening in association with magma accumulation in the crust. Fluids for mineralization are released as the crust thickens, and hydrous, lower crustal, amphibole-bearing mineral assemblages that were stable during earlier stages of crustal thickening break down to dryer, more garnet-bearing ones. Evidence for this process comes from trace-element signatures of preto postmineralization magmas that show a progression from equilibration with intermediate pressure amphibole-bearing residual mineral assemblages to higher pressure garnet-bearing ones. Mineralization over the shallowing subduction zone in central Chile (28°–33°S) is followed by cessation of arc volcanism or migration of the arc front away from the trench. Mineralization in the central Altiplano-Puna region (21°–24°S) formed above a formerly flat subduction zone as volcanism was reinitiating. Thus, hydration and crustal thickening associated with transitions in and out of flat-slab subduction conditions are fundamental controls on formation of these major ore deposits. INTRODUCTION Some of the world’s richest and largest copper and gold deposits are associated with Miocene magmatism in the central Andes. This paper reviews how the formation of major ore deposits between 22° and 34°S can be linked to the late Cenozoic magmatic and tectonic response of the mantle and lower crust to the formation and subsequent steepening of shallow subduction zones (Figs. 1 and 2). Mineral districts discussed are the El Central Andean Ore

Magmatic evidence for Neogene lithospheric evolution of the central Andean “flat-slab” between 30°S and 32°S

Abstract Geochemical data from Andean Miocene magmas erupted through the now volcanically-inactive “flat-slab” between 30°S to 32°S, coupled with geological and geophysical data, illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone. Pb, Sr and Nd isotopic and trace-element data show that Main Cordilleran, Precordilleran and Pampean magmas were contaminated in both the mantle and crust, and that the nature of the contaminants varied in time and space, reflecting tectonic events. Contaminants included both “enriched” (high LIL-element, high Sr and Pb, and low Nd isotopic ratios) and “depleted” (low LILE-element, low Sr and Pb, and high Nd isotopic ratios) types. In the western region, Main Cordilleran earlier Miocene lavas had contaminants with less “enriched” signatures than later Miocene lavas. Progressive “enrichment” is attributed to: (a) increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge; and (b) contamination in a thickening Main Cordilleran lower crust whose composition was progressively “enriched”. This “enrichment” occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging, heating and deformation of crust from beneath the shortening Precordillera thrust belt to the east. Further east, magmas erupted through back-arc crust have more “depleted” signatures. Those erupted in the central part through the evolving Precordilleran thrust belt were contaminated by an older, thinner Grenville (∼ 1100 Ma) basement whose “depleted” signature is unique among Central Andean terranes. Late Miocene Pocho lavas erupted further east in conjunction with uplift of the Sierras Pampeanas show “enrichment” through time. Arguably, these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west, and progressively incorporated into the convecting asthenosphere. First-order mass balance calculations comparing the lithospheric geometry proposed for 20 Ma with that today suggests that ∼ 60% of the lithosphere within 600 km of the trench at 20 Ma has been lost. This lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab, and thickening and shortening of the crust. Continental lithosphere recycled into the asthenosphere could contribute to the sources of the “enriched” EM1 and EM2 mantle components in oceanic island basalts.

Neogene Patagonian plateau lavas: Continental magmas associated with ridge collision at the Chile Triple Junction

Extensive Neogene Patagonian plateau lavas (46.5° to 49.5°S) southeast of the modern Chile Triple Junction can be related to opening of asthenospheric “slab windows” associated with collisions of Chile Rise segments with the Chile Trench at ≈ 12 Ma and 6 Ma. Support comes from 26 new total-fusion, whole rock 40Ar/39Ar ages and geochemical data from back arc plateau lavas. In most localities, plateau lava sequences consist of voluminous, tholeiitic main-plateau flows overlain by less voluminous, 2 to 5 million year younger, alkalic postplateau flows. Northeast of where the ridge collided at ≈12 Ma, most lavas are syncollisional or postcollisional in age, with eruptions of both sequences migrating northeastward at 50 to 70 km/Ma. Plateau lavas have ages from 12 to 7 Ma in the western back arc and from 5 to 2 Ma farther to the northeast. Trace element and isotopic data indicate main-plateau lavas formed as larger percentage melts of a garnet-bearing, oceanic island basalt (OIB) -like mantle than postplateau lavas. The highest percentage melts erupted in the western and central plateaus. In a migrating slab window model, main-plateau lavas can be explained as melts that formed as upwelling, subslab asthenosphere which flowed around the trailing edge of the descending Nazca Plate and then interacted with subduction-altered asthenospheric wedge and continental lithosphere. Alkaline, postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window. Thermal problems of high-pressure melt generation of anhydrous mantle can be explained by volatiles (H2O and CO2) introduced by the subduction process into slab window source region(s). An OIB-like, rather than a mid-ocean ridge basalt (MORB) -like source region, and the lack of magmatism northeast of where ridge collision occurred at ≈13 to 14 Ma can be explained by entrainment of “weak” plume(s) or regional variations in an ambient, OIB-like asthenosphere.

Late Paleozoic to Triassic evolution of the Gondwana margin: Evidence from Chilean Frontal Cordilleran batholiths (28°S to 31°S)

Upper Paleozoic to Triassic Chilean granitoids in the Andean Frontal Cordillera between 28°S and 31°S record crustal and mantle conditions at the Gondwana margin during the final assembly and initial breakup of the Pangea supercontinent. This period overlaps the end of Paleozoic terrane accretion and precedes Andean subduction. Integration of new trace-element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the Gondwana margin. In the model, the Carboniferous to Early Permian is a period of oblique convergence. Associated Elqui complex granitoids are diverse. Those in the Guanta and Montosa units are predominantly related to subduction processes, whereas those in the Cochiguas and El Volcan units are dominated by melting of the subduction complex and older crust. Progressive oblique collision of the last pre-Pangea terrane (Equis) along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the Elqui complex. Subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt, which intruded and melted the crust, producing the post-collisional Ingaguas complex. The Los Carricitos granitoids formed in thickened crust, whereas the Chollay, El Colorado, and El Leon units formed in thinner crust. The Ingaguas complex is part of the Choiyoi granite-rhyolite province, whose formation, similar to that of other Gondwana silicic provinces, was probably accentuated by anomalously hot upper mantle associated with the Pangea supercontinent.

Revised age of Aleutian Island Arc formation implies high rate of magma production

Radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the Aleutian Island Arc provides insight into the timing of arc formation in the middle Eocene. Twenty-eight 40 Ar/ 39 Ar ages constrain the duration of arc magmatism to the last 46 m.y. Basaltic lavas from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave an isochron age of 37.4 ± 0.6 Ma, which is 12-17 m.y. younger than a widely cited age of 55-50 Ma. Three main pulses of arc-wide magmatism occurred at 38-29, 16-11, and 6-0 Ma, which coincide with periods of intense magmatism in other western Pacific island arcs. Using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m.y., we calculate a time-averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude.