Sherman H. Bloomer

Subduction zone infancy: Examples from the Eocene Izu-Bonin-Mariana and Jurassic California arcs

A new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the Izu-Bonin-Mariana (IBM) arc system and then applied to Late Jurassic ophiolites of Cailfornia. The model accounts for several key observations about the earliest stages in the evolution of the IBM system: (1) subduction nucleated along an active transform boundary, which separated younger, less-dense lithosphere in the west from older, more-dense lithosphere to the east; (2) initial arc magmatic activity occupied a much broader zone than existed later; (3) initial magmatism extended up to the modern trench, over a region now characterized by subnormal heat flow; (4) early are magmatism was characterized by depleted (tholeiitic) and ultra-depleted (boninitic) magmas, indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth; (5) early arc magmatism was strongly extensional, with crust forming in a manner similar to slow-spreading ridges; and (6) crust production rates were 120 to 180 km 3 /km-Ma, several times greater than for mature arc systems. These observations require that the earliest stages of subduction involve rapid retreat of the trench; we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault. This resulted in strong extension and thinning of younger, more buoyant lithosphere to the west. This extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere. Addition of water and asthenospheric upwelling led to catastrophic melting, which continued until lithosphere subsidence was replaced by lithospheric subduction . Application of the subduction-zone infancy model to the Late Jurassic ophiolites of California provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger Sierran magmatic arc and the Franciscan subduction complex, provides a mechanism for the formation and subsidence of the Great Valley forearc basin, and explains the limited duration of high-T, high-P metamorphism experienced by Franciscan melanges.

MORB mantle and subduction components interact to generate basalts in the southern Mariana Trough back-arc basin

We report the results of the first geochemical and isotopic survey of basaltic glasses dredged along the spreading ridge of the southern Mariana Trough (SMT; 15–17°N). This ridge is divided into two segments that have different axial depths, major and trace element compositions, water contents, and isotopic compositions of Sr, Nd, and Pb. Glasses from the shallower, northern segment (N-SMT; 16–17°N) are OL- and QZ-tholeiites that have compositions consistent with a higher degree of mantle melting relative to that of the OL tholeiites from the southern ridge segment (S-SMT; 15–16°N). The N-SMT glasses are similar to basalts erupted near 18°N in the Mariana Trough that have been the focus of previous studies. The more extensive melting inferred for the N-SMT correlates well with higher abundances of water and relative abundances of large ion lithophile and light Rare Earth elements that indicate involvement of a subduction component. The southern ridge segment is deeper and erupts compositions characteristic of lower degrees of melting; this correlates well with a lower proportion of the subduction component, including a suite that is indistinguishable from MORB. The strong correlation between degree of melting, water contents, and LIL elements indicates that hydrous fluxing as well as adiabatic decompression control melting of MORB-like mantle beneath back-arc basins. Details regarding the nature of this hydrous fluxing agent are not known, but it could be water-rich melts related to behind-the-arc volcanoes. These melts may be diverted by the back-arc convective regime, to become entrained in the zone of adiabatic upwelling, where they further stimulate melting.

Enriched back-arc basin basalts from the northern Mariana Trough: implications for the magmatic evolution of back-arc basins

Abstract The composition of basalts erupted at the earliest stages in the evolution of a back-arc basin permit unique insights into the composition and structure of the sub-arc mantle. We report major and trace element chemical data and O-, Sr-, Nd-, and Pb- isotopic analyses for basalts recovered from four dredge hauls and one ALVIN dive in the northern Mariana Trough near 22°N. The petrography and major element chemistry of these basalts (MTB-22) are similar to tholeiites from the widest part of the Trough, near 18°N (MTB-18), except that MTB-22 have slightly more K 2 O and slightly less TiO 2 . The trace element data exhibit a very strong arc signature in MTB-22, including elevated K, Rb, Sr, Ba, and LREE contents; relatively lowK/Ba and highBa/La andSr/Nd. The Sr- and Nd- isotopic data plot in a field displaced from that of MTB-18 towards Mariana arc lavas, and the Pb-isotopic composition of MTB-22 is indistinguishable from Mariana arc lavas and much more homogeneous than MTB-18. Mixing of 50–90% Mariana arc component with a MORB component is hypothesized. We cannot determine whether this resulted from physical mixing of arc mantle and MORB mantle, or whether the arc component is introduced by metasomatism of MORB-like mantle by fluids released from the subducted lithosphere. The strong arc signature in back-arc melts from the Mariana Trough at 22°N, where the back-arc basin is narrow, supports general models for back-arc basin evolution whereby early back-arc basin basalts have a strong arc component which diminishes in importance relative to MORB as the back-arc basin widens.

Petrology and geochemistry of boninite series volcanic rocks from the Mariana trench

Boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the Mariana Trench. These hauls included olivine boninites, boninites, boninitic andesites and boninitic dacites, as well as island arc tholeiitic basalts and andesites. The boninite series volcanics range from 52 to 68% SiO2, and are characterized by very low abundances of high-field-strength cations and heavy-rare-earth elements. Boninites and olivine boninites have phenocrysts of olivine and orthopyroxene, the andesites phenocrysts of orthopyroxene and clinopyroxene, and the dacites orthopyroxene, clinopyroxene, and plagioclase. Most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine, orthopyroxene, clinopyroxene, and plagioclase in the proportions 2.5∶4∶1∶2, leaving 47% residual liquid. The fractionation must be in part open-system: reverse zoned phenocrysts, resorbed olivine and plagioclase xenocrysts, and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation. Two boninitic magma stems can be identified, with similar high-field-strength element abundances, but different amounts of Ca, Na, Al and light-rare-earth elements. There is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites. The compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid. The Mariana forearc boninite series lacks a strong iron enrichment, but produces andesites with lower Ti, Al and Y/Zr, and higher Mg, Ni and Cr than typical calcalkaline arc andesites and dacites. Boninites in the Mariana system were erupted only in the earliest phases of subduction zone activity.

The source of the subduction component in convergent margin magmas: Trace element and radiogenic isotope evidence from Eocene boninites, Mariana forearc

Boninites are generally accepted as being melts from mixtures of depleted harzburgite and a water- and incompatible element-enriched component thought to be derived from the subducted plate (the “subduction component”). From calculations in this study, Mariana boninites are inferred to obtain 70–90% of Sr, 60–95% of Pb, and 0–80% of Nd from the subduction component, and so provide unique insights into the composition and source of this material as sampled early in the development of the arc. Nd-, Pb-, and Sr-isotopic compositions of Eocene boninites from three dredge sites in the Mariana forearc indicate that this subduction component is isotopically indistinguishable from mantle sources responsible for the generation of typical, northern hemisphere ocean-island basalt. Initial 87Sr86Sr (0.7032 to 0.7034), ϵ-Nd- (+7.4 to +9.2), and Pb-isotopic compositions (206Pb204Pb = 18.4 to 18.9,207Pb204Pb = 15.45 to 15.55, 208Pb204Pb = 37.92 to 38.45; Δ74 = −4 to +1.6; Δ84 = -10 to +12) fall within the Sr-Nd mantle array and along the NHRL for Pb-isotopic compositions. The values for the Eocene boninites are very similar to those of modern Mariana arc lavas, indicating that the subduction component is isotopically homogeneous in time and space. If the depleted endmember in boninite petrogenesis is assumed to be a MORB-source, subducted sediments cannot be significant sources of the subduction component. Instead, the subduction component identified for these boninites must have been derived from dehydration of subducted basaltic crust or via re-equilibration of fluids—and cations—released from the dehydrating slab with the overlying mantle wedge.

Nd- and Sr-isotopic compositions of lavas from the northern Mariana and southern Volcano arcs: implications for the origin of island arc melts

Nd- and Sr-isotopic data are reported for lavas from 23 submarine and 3 subaerial volcanoes in the northern Mariana and southern Volcano arcs. Values of εNd range from +2.4 to +9.5 whereas 87Sr/86Sr ranges from 0.70319 to 0.70392; these vary systematically between and sometimes within arc segments. The Nd-and Sr-isotopic compositions fall in the field of ocean island basalt (OIB) and extend along the mantle array. Lavas from the Volcano arc, Mariana Central Island Province and the southern part of the Northern Seamount Province have εNd to +10 and 87Sr/86Sr=0.7032 to 0.7039. These are often slightly displaced toward higher 87Sr/86Sr at similar εNd. In contrast, those lavas from the northern part of the Mariana Northern Seamount Province as far north as Iwo Jima show OIB isotopic characteristics, with εNd and 87Sr/86Sr=0.7035 to 0.7039. Plots of 87Sr/86Sr and εNd versus Ba/La and (La/Yb)n support a model in which melts from the Mariana and Volcano arcs are derived by mixing of OIB-type mantle (or melts therefrom) and a metasomatized MORB-type mantle (or melts therefrom). An alternate interpretation is that anomalous trends on the plots of Nd- and Sr-isotopic composition versus incompatible-element ratios, found in some S-NSP lavas, suggest that the addition of a sedimentary component may be locally superimposed on the two-component mixing of mantle end-members.

Physical volcanology of the submarine Mariana and Volcano Arcs

Narrow-beam maps, selected dredge samplings, and surveys of the Mariana and Volcano Arcs identify 42 submarine volcanos. Observed activity and sample characteristics indicate 22 of these to be active or dormant. Edifices in the Volcano Arc are larger than most of the Mariana Arc edifices, more irregularly shaped with numerous subsidiary cones, and regularly spaced at 50–70 km. Volcanos in the Mariana Arc tend to be simple cones. Sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110° counterclockwise from that trend, suggesting a strong fault control on the distribution of arc magmas. Volcanos in the Mariana Arc are generally developed west of the frontal arc ridge, on rifted frontal arc crust or new back-arc basin crust. Volcanos in the central Mariana Arc are usually subaerial, large (> 500 km3), and spaced about 50–70 km apart. Those in the northern and southern Marianas are largely submarine, closer together, and generally less than 500 km3 in volume. There is a shoaling of the arc basement around Iwo Jima, accompanied by the appearance of incompatible-element enriched lavas with alkalic affinities. The larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos. If the magma supply (estimated at 10–20 km3/km of arc per million years at 18° N) has been relatively constant along the Mariana Arc, we can infer a possible evolutionary sequence for arc volcanos from small, irregularly spaced edifices to large (over 1000 km3) edifices spaced at 50–70 km. The volcano distribution and basal depths are consistent with the hypothesis of back-arc propagation into the Volcano Arc.

Petrology and Geochemistry of Igneous Rocks from the Tonga Trench: A Non-Accreting Plate Boundary

Petrologic and geochemical examination of a varied suite of intermediate, mafic, and ultramafic rocks dredged from the deep flanks of the Tonga Trench between 20°S and 21°S show that the landward slope has not developed by accretion of material from the subducted Pacific plate. The lowermost trench slopes (> 9000 m) are part of a graben in the Mesozoic Pacific plate; the west and east walls of this graben expose normal- and enriched-type ocean-ridge basalts. The distribution of recovered rock types suggests that the shoaler (< 9000 m) nearshore flank is crudely layered from peridotite (up to 8500 m), to gabbro (to 7000 m), to volcanic rocks (7000-5000 m). Peridotites are fresh to moderately serpentinized harzburgites, with some dunite and minor lherzolite. They are distinctly more depleted in clinopyroxene and have more magnesian mineral compositions than are characteristic of tectonite periedotites from ocean-ridges, fracture zones, and many ophiolites. Volcanics from the upper nearshore slope are basalts, andesites, and dacites. Unlike the N- and E-type basalts being subducted, they are most similar to low-Ti, low-Ba arc volcanics from the Lau Ridge and Mariana forearc. Overall, the coherent 4 + km crustal section of the nearshore flank exposed above the structural plate boundary at 9000+ m is geochemically unlike crust generated in mid-ocean. It may be primitive island-arc crust, older oceanic crust modified by island-arc volcanism, or atypical ocean-ridge crust upon which the Tonga arc was constructed. There is no evidence of accretion of material like that being subducted-non-accretion or tectonic erosion are inferred to be the principal processes which have shaped this margin.

Shoshonitic magmas in nascent arcs: New evidence from submarine volcanoes in the northern Marianas

Volcanoes in the northern Mariana arc between Uracas (lat 20°N) and Minami Iwo Jima (24°N) are very active yet entirely submarine. In contrast to the predominantly low-K basaltic magmas of the central Mariana arc, the northern Mariana arc is dominated by more siliceous melts in the south and by shoshonites in the north. The northern arc melts have enrichments in Ba (<800 ppm), Rb (<70 ppm), Sr (<1000 ppm), Ce (<50 ppm), and (Ce/Yb)n (<24) which increase to the north as far as Iwo Jima. Lavas from volcanoes north of Iwo Jima lack these enrichments and are indistinguishable from those of the central Maranas. The shoshonites are unusual in occurring along the magmatic front of a primitive, intra-oceanic arc. We hypothesize that they represent the reconstruction of a magmatic arc following melting of enriched mantle due to the propagation of the Mariana Trough spreading center northward through the Volcano arc. Shoshonites thus may characterize the initial stages of arc construction after an episode of back-arc rifting and need not be restricted to the mature stages of arc evolution. This situation contrasts with subduction-zone initiation, where first melts may be boninites or low-K tholeiites. These differing initial melts converge toward tholeiitic and calc-alkaline compositions as arcs evolve.

Submarine arc volcanism in the southern Mariana Arc as an ophiolite analogue

Supra-subduction zone (SSZ) ophiolites are being recognized more and more frequently in the geologic record. Submarine volcanism in intra-oceanic arcs is one tectonic setting in which such ophiolites might form. The Southern Seamount Province of the Mariana Arc represents the best documented example of an active intra-oceanic submarine arc which can serve as a modern analogue for some SSZ ophiolites. The Southern Seamount Province comprises nine submarine volcanoes, at least seven of which are active or dormant. These edifices have erupted plagioclase-clinopyroxene-olivine-phyric basalts, andesites, dacites and dacitic pumices. These lavas are similar to those of the subaerial edifices in the central Marianas (SiO2 = 49–70%, TiO2 = 0.4−1.3%, MgO = 1.2–5.9%, Ba = 140–450 ppm, 87Sr/86Sr = 0.70346−0.70354), but are somewhat more enriched in the light rare-earth elements. The geochemical characteristics of subaerial Mariana Arc lavas are preserved in the submarine lavas, and the SSP lavas are slightly enriched in incompatible elements relative to those of the subaerial edifices. We are ignorant of the composition of any lavas erupted between the principal edifices, which may be expected to dominate the volcanic section of SSZ ophiolites. Nevertheless, the characteristic high ratio LIL/HFSC, low Ti, Ni, and Cr, and diagnostic mineral compositions (calcic plagioclase, relatively Fe-rich olivine) are likely to be preserved in unaltered submarine arc lavas. The Mariana active arc is built on back-arc, or thinned frontal arc, crust and any ophiolite formed from such an arc is likely to exhibit complex intrusive and eruptive relationships of plutonic, hypabyssal, and volcanic rocks of back-arc, frontal arc, and active arc origin. Such complicated intrusive relationships, if preserved in structurally disrupted ophiolites, will make it difficult to reconstruct the geologic history of an SSZ ophiolite.