Andrew D. Saunders

Transverse geochemical variations across the Antarctic Peninsula: Implications for the genesis of calc-alkaline magmas

Magnetic activity throughout the Antarctic Peninsula and the South Shetland Islands has been dominantly of a calc-alkaline nature for the last 200 Ma. Chemically, the plutonic and volcanic products are typical of a continental margin magmatic arc, similar to those from western South America. Within any one area, volcanic and plutonic rocks are compositionally indistinguishable, and all magmatic products show increasing SiO2, and increasing K/Si, Rb/Si, Th/Si and to a lesser extent Ce/Si and La/Si ratios away from the proposed trench axis. The calc-alkaline basaltic compositions also have high large ion lithophile (LIL; e.g. K, Rb, Th)/high field strength (HFS; e.g. Zr, Nb, Ti) ratios relative to non-orogenic counterparts, and increasing LIL/HFS element ratios with increasing fractionation. It is proposed that the high LIL/HFS element ratios in basaltic and andesitic melts are primary features due to dehydration processes with the subducted slab and to fractionation of minor mineral phases from the melt. The increasing LIL/HFS element ratios in more acid rocks are probably due to removal of minor mineral phases from the melt. Although zone refining may contribute to the spatial variations across the peninsula, we have proposed that an enriched subcontinental mantle provides a viable alternative source for the observed K-h variations and for the increased LIL-element contents found in continental margin calc-alkaline magmas.

The geochemistry of basalts from a back-arc spreading centre in the East Scotia Sea

Rapid sea floor spreading has taken place over the last 8 Myr behind the South Sandwich island arc, producing a regular set of magnetic lineations. Suites of fresh basalts have been dredged from four widely separated localities along the spreading axis. Dredges 20 and 23 yielded sub-alkaline olivine tholeiites, dredge 22 recovered vesicular tholeiites with minor normative olivine, while dredge 24 contained a fractionated suite of highly vesicular quartz-normative basalts with higher FeMg. The concentrations of the incompatible elements Ti, P, Zr, Hf, Nb, Ta, Y and the REE increase systematically from dredge 24 through dredges 22 and 20 to dredge 23 and there is a comparable increase in CeNYbN. Quantitative modelling suggests that all the basalts can be derived from an essentially similar mantle source (with respect to these elements) through varying degrees of partial melting, but involving some residual clinopyroxene. Basalts from dredge 24 have unusually low concentrations of Ti, P, Zr, Nb, Y, REE and Ni, similar to the values in arc tholeiites, and the more primitive dredge 24 liquids seem to have been generated through high degrees of partial melting (~ 30%) leaving a dunitic residue. Transitional arc tholeiite characteristics are also apparent in the relatively high K, Rb, Ba contents and 87Sr86Sr ratios of dredge 24 and 22 basalts, though Nd isotope ratios are uniform. It is considered that fluids derived from the dehydrating subducted slab may have locally penetrated the source regions of the back-arc basalts, carrying K, Rb, Ba and seawater-enriched 87Sr, and producing conditions of magma generation similar to that of arc tholeiites. However, it is unlikely that the sources for these and other marginal basin basalts differ fundamentally from the range of mantle sources feeding normal mid-ocean ridges.

GEOCHEMISTRY OF BASALTS DRILLED IN THE NORTH-ATLANTIC BY IPOD LEG-49 - IMPLICATIONS FOR MANTLE HETEROGENEITY

Abstract IPOD Leg 49 recovered basalts from 9 holes at 7 sites along 3 transects across the Mid-Atlantic Ridge: 63°N (Reykjanes), 45°N and 36°N (FAMOUS area). This has provided further information on the nature of mantle heterogeneity in the North Atlantic by enabling studies to be made of the variation of basalt composition with depth and with time near critical areas (Iceland and the Azores) where deep mantle plumes are thought to exist. Over 150 samples have been analysed for up to 40 major and trace elements and the results used to place constraints on the petrogenesis of the erupted basalts and hence on the geochemical nature of their source regions. It is apparent that few of the recovered basalts have the geochemical characteristics of typical “depleted” midocean ridge basalts (MORB). An unusually wide range of basalt compositions may be erupted at a single site: the range of rare earth patterns within the short section cored at Site 413, for instance, encompasses the total variation of REE patterns previously reported from the FAMOUS area. Nevertheless it is possible to account for most of the compositional variation at a single site by partial melting processes (including dynamic melting) and fractional crystallization. Partial melting mechanisms seem to be the dominant processes relating basalt compositions, particularly at 36°N and 45°N, suggesting that long-lived sub-axial magma chambers may not be a consistent feature of the slow-spreading Mid-Atlantic Ridge. Comparisons of basalts erupted at the same ridge segment for periods of the order of 35 m.y. (now lying along the same mantle flow line) do show some significant inter-site differences in Rb/Sr, Ce/Yb, 87 Sr/ 86 Sr, etc., which cannot be accounted for by fractionation mechanisms and which must reflect heterogeneities in the mantle source. However when hygromagmatophile (HYG) trace element levels and ratios are considered, it is the constancy or consistency of these HYG ratios which is the more remarkable, implying that the mantle source feeding a particular ridge segment was uniform with respect to these elements for periods of the order of 35 m.y. and probably since the opening of the Atlantic. Yet these HYG element ratios at 63°N are very different from those at 45°N and 36°N and significantly different from the values at 22°N and in “MORB”. The observed variations are difficult to reconcile with current concepts of mantle plumes and binary mixing models. The mantle is certainly heterogeneous, but there is not simply an “enriched” and a “depleted” source, but rather a range of sources heterogeneous on different scales for different elements — to an extent and volume depending on previous depletion/enrichment events. HYG element ratios offer the best method of defining compositionally different mantle segments since they are little modified by the fractionation processes associated with basalt generation.

A geochemical study of magmatism associated with the initial stages of back-arc spreading

Bransfield Strait is a narrow basin separating the South Shetland Islands from the Antarctic Peninsula and is attributed to recent back-arc extension behind the South Shetland volcanic arc. The volcanic islands of Deception and Bridgeman are situated close to the axis of spreading, whereas Penguin Island lies slightly to the north of this axis. The mineralogy, petrology and geochemistry of the lavas of the three volcanoes have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading.Deception Island lavas range from olivine basalt to dacite, and all are highly sodic, with high Na/K, K/Rb, Ba/Rb and Zr/Nb ratios and with CeN/YbN = 2. Incompatible elements increase systematically between basalt and rhyodacite, while Sr decreases, suggesting that fractional crystallisation is the dominant process relating lava compositions. The rhyodacites have high concentrations of Zr, Y and the REE and negative Eu anomalies and are compositionally similar to oceanic plagiogranite. Bridgeman Island lavas are mostly basaltic andesites, but the levels of many incompatible elements, including REE, are significantly lower than those of Deception lavas, although CeN/YbN ratios and 87Sr/86Sr ratios (0.7035) are the same. Penguin Island lavas are magnesian, mildly alkaline olivine basalts with a small range of composition that can be accommodated by fractional crystallisation of olivine, clinopyroxene and/or chromite. Penguin lavas have higher 87Sr/86Sr (0.7039) and CeN/ YbN (4) ratios than Deception and Bridgeman lavas. The Rb/Sr ratios of Deception and Penguin basalts (ca. 0.01) are much too low to account for their present 87Sr/86Sr ratios.Modelling suggests that the source regions of the lavas of the three volcanoes share many geochemical features, but there are also some significant differences, which probably reflects the complex nature of the mantle under an active island arc combined with complex melting relationships attending the initial stages of back-arc spreading. Favoured models suggest that Bridgeman lavas represent 10–20% melting and the more primitive Deception lavas 5–10% melting of spinel-peridotite, whereas Penguin lavas represent less then 5% melting of a garnet-peridotite source. The mantle source for Bridgeman lavas seems to have undergone short-term enrichment in K, Rb and Ba, possibly resulting from dewatering of the subducted slab. Hydrous melting conditions may also account for the more siliceous, high-alumina nature and low trace element contents of Bridgeman lavas.

Geochemistry of Mesozoic marginal basin floor igneous rocks from southern Chile

Extension behind a Late Jurassic continental margin volcanic arc in southern Chile caused rifting and the development of a narrow marginal basin floored by oceanic crust. Extension ceased and the basin was closed and uplifted in mid-Cretaceous time, so the basin floor is now exposed as the upper part of an autochthonous ophiolite complex composed of gabbros, sheeted dikes, and pillow lavas, with minor plagiogranite and associated siliceous dikes. Many of the rocks are altered. The metamorphic grade increases from zeolite or greenschist facies in the pillow lavas to amphibolite facies in the gabbros, but the maximum intensity of recrystallization occurs in the sheeted dike unit and is associated with loss of Rb and K and increasing K/Rb ratio, contrasting with the effects produced by low-temperature alteration of basalts by sea water. Metamorphic effects seem to be related to hydrothermal convective systems operating at the spreading axis at the time of basin formation. Geochemically, the rocks have affinities with mid-oceanic ridge basalts, but K, Rb, and Ba contents and Ba/Sr and Ce/Yb ratios are higher and K/Rb ratios are lower in the least altered rocks than in mid-oceanic ridge basalts. Similar features are apparent in some other marginal basin basalts. Fractionation trends are tholeiitic, the mafic rocks displaying a wide range of Fe/Mg ratios (0.9 to 5.2) but without any concomitant silica enrichment. Rare-earth elements, TiO 2 , and Zr correlate positively and Cr and Ni negatively with Fe/Mg, while the gabbros have lower contents of some incompatible elements as a result of their cumulate nature. The leucocratic rocks within the mafic complex have been derived from two distinct sources. Some trondhjemites and granophyres have compositions indicating derivation by refusion of continental material bordering the mafic complex. The plagiogranites, however, have a distinctive geochemistry, consistent with an origin by high-level differentiation of the mafic magmas. Such rocks, normally lying in or just below the sheeted dike unit, may be a common if minor component of oceanic crust.

Nature of mantle heterogeneity in the North Atlantic: evidence from deep sea drilling

Studies of dredged and drilled samples from the North Atlantic ocean have revealed that basalts with a wide range of major and trace element compositions have been generated at the Mid-Atlantic Ridge (M.A.R.). Many of the basalts erupted between latitudes 30° and 70° N do not have the geochemical characteristics of normal mid-ocean ridge basalts (m.o.r.b.) depleted in the more-hygromagmatophile (hyg.) elements. Drilling along mantle flow lines transverse to the ridge has shown that different segments of the M.A.R. have produced basalts with a distinct compositional range for tens of millions of years. As more data have become available, the nature and scale of this variation have been established and tighter constraints can now be placed on the petrogenetic processes involved. The rare earth elements are used to test quantitatively the effects of open and closed system fractional crystallization, equilibrium partial melting (including continuous melting), zone refining and mantle mixing processes on basalt chemistry. When evaluated in terms of the more-hyg. elements, the results show that major heterogeneities must exist in the mantle sources feeding the M.A.R. Ratios of many of the more-hyg. elements remain consistent in space and time in basalts erupted at a particular ridge segment, but vary widely between different ridge segments. These ratios are not significantly modified by the processes of basalt generation. The hyg. element relations provide a major constraint on the nature of heterogeneity in the Earth’s mantle and the processes producing it. The mantle sources of anomalous ridge segments can be best explained in terms of variable veining of a hyg. element depleted host by a hyg. element enriched liquid or fluid generated by very small degrees of partial melting. Such incipient melting, as well as subduction zone processes, may be viable mechanisms for changing hyg. element ratios in the mantle source regions on the scale observed. These processes can be integrated into a model for mantle evolution which involves (1) upward migration of incipient melts to provide a hyg. element enriched source for alkali basalts and a hyg. element depleted source for normal m.o.r.b., and (2) extraction of continental crust and recycling of the depleted residue into the mantle at subduction zones. Also, some recycling of continental material into the mantle may be required to explain Pb isotope patterns.

VOLATILES IN SUBMARINE VOLCANIC-ROCKS FROM THE SPREADING AXIS OF THE EAST SCOTIA SEA BACK-ARC BASIN

Abstract The volatile content of glassy pillow rims from East Scotia Sea back-arc basin (BAB) lavas are unlike those of mid-ocean ridge (MOR) pillow-rim glasses, although non-volatile compositions of the two rock groups overlap. The East Scotia Sea samples have three to ten times greater water contents and nearly twice the average CO 2 and Cl contents of MOR samples; F contents are similar. S contents are only one-third those from MOR samples. H 2 O and CO 2 contents of glassy pillow rims from Mariana island arc andesites are similar to those in the BAB lavas studied. Nevertheless, volatiles in the East Scotia Sea BAB magmas are probably not directly derived from the subducted slab, because there is no seismic evidence that the slab extends within 200 km of the spreading axis of the East Scotia Sea. Available data do not preclude the possibility that the magmas were contaminated by seawater prior to eruption or that the mantle under the East Scotia Sea spreading center is volatile-rich. The volatiles may have been added to the mantle during an earlier period of subduction, perhaps during the initial formation of the East Scotia Sea basin.

Trace Element Constraints on the Origin of Cordilleran Batholiths

The problem of the origin of cordilleran batholiths cannot, because of their immense volume, be separated from the origin of the continental crust itself. Despite arguments as to when and how the crust was generated, the extent of continental reworking and mobilization and the importance of sediment recycling (see Fyfe, 1978, 1979; Brown, this volume), there can be little doubt from isotopic studies (Moorbath, 1977, 1978; McCulloch and Wasserburg, 1978; Hamilton et al., 1979) that large areas of continental crust represent new additions of sialic material from the mantle system at various periods. Tarney (1976) and Windley and Smith (1976) drew attention to the broad compositional similarities between Archaean continental crust and the cordilleran batholiths of North and South America. Tonalite is a dominant component in both — this is particularly so in the more deeply eroded parts of the Andean cordillera, where the plutonic rocks frequently have a distinct fabric and may even be foliated. Whether the crustal generation processes in the Archaean exactly paralleled those in the modern Andes, however, remains to be determined.