Geochemistry of basalts at DSDP Leg 73 Holes

During Deep Sea Drilling Project Leg 73 (South Atlantic), basaltic pillow lava, flows, and sills were encountered in Holes 519A, 520, 522B, and 524. Paleomagnetic data indicate that the basalts from Holes 519A (magnetic Anomaly 51) and 522B (Anomaly 16) have ages of about 12 m.y. and about 38 m.y., respectively. The major- and trace- (including rare-earth-) element characteristics of the Hole 519A basalts (a total of 27 m) demonstrate that these basalts are typical normal-type mid-ocean-ridge basalts (N-type MORB). In composition the basalts overlap olivine tholeiites from other normal Mid-Atlantic Ridge segments. Both the spectra of incompatible, or less-hygromagmatophile elements (such as Ti, V, Y, and Zr) and REE abundances indicate that these basalts are the result of a low-pressure fractionation of olivine, spinel, and Plagioclase prior to eruption. In Hole 520 only 1.7 m of basalt were recovered from a total drilling depth of 10.5 m. These pillow basalts crystallized from fairly evolved (N-type MORB) tholeiitic melts. In total, 19 m of basaltic pillow lavas and flows were penetrated in Hole 522B. Thirteen cooling units were distinguished on the basis of glassy margins and fine quench textures. In contrast to Holes 519A and 520, the basalts of the Hole 522B ridge section can be divided into two major groups of tholeiites: (1) Cooling Units 1 through 12 and (2) Cooling Unit 13. The basalts in this ridge section are also N-type MORBs but are generally more differentiated than those of Holes 519A and 520. The lowermost basalts (Cooling Unit 13) have the most primitive composition and make up a compositional group distinct from the more evolved basalts in the twelve units above it. Hole 524 was drilled on the south flank of the Walvis Ridge and thus provided samples from a more complex part of the South Atlantic seafloor. Three different basaltic rock suites, interlayered with volcanic detrital sediments, were encountered. The rock suites are, from top to bottom, an alkali basaltic pillow lava; a 16-m-thick alkaline diabase sill with an age of about 65 m.y. (according to K-Ar dating and planktonic foraminifers); and a second sill that is approximately 9 m thick, about 74 m.y. in age, and tholeiitic in composition, thus contrasting strongly with the overlying alkaline rocks. The alkali basalts of Hole 524 show chemical characteristics that are very similar to the basaltic lavas of the Tristan da Cunha group volcanoes, which are located approximately 400 km east of the Mid-Atlantic Ridge crest. Thus, the Walvis Ridge may plausibly be interpreted as a line of hot-spot alkaline volcanoes.

Geochemical composition of the Trincheira Complex in the southwestern Amazon craton, Brazil

We document the first-known Mesoproterozoic ophiolite from the southwestern part of the Amazon craton, corresponding to the Trincheira Complex of Calymmian age, and propose a tectonic model that explains many previously enigmatic features of the Precambrian history of this key craton, and discuss its role in the reconstruction of the Columbia supercontinent. The complex comprises extrusive rocks (fine-grained amphibolites derived from massive and pillowed basalts), mafic-ultramafic intrusive rocks, chert, banded iron formation (BIFs), pelites, psammitic and a smaller proportion of calc-silicate rocks. This sequence was deformed, metasomatized and metamorphosed during the development of the Alto Guaporé Belt, a Mesoproterozoic accretionary orogen. The rocks were deformed by a single tectonic event, which included isoclinal folding and metamorphism of the granulite-amphibolite facies. Layered magmatic structures were preserved in areas of low strain, including amygdaloidal and cumulate structures. Metamorphism was pervasive and reached temperatures of 780-853°C in mafic granulites and 680-720°C in amphibolites under an overall pressure of 6.8 kbar. The geochemical composition of the extrusive and intrusive rocks indicates that all noncumulus mafic-ultramafic rocks are tholeiitic basalts. The mafic-ultramafic rocks display moderately to strongly fractionation of light rare earth elements (LREE), near-flat heavy rare earth elements (HREE) patterns and moderate to strong negative high field strength elements (HFSE) anomalies (especially Nb), a geochemical signature typical of subduction zones. The lowest units of mafic granulites and porphyroblastic amphibolites in the Trincheira ophiolite are similar to the modern mid-ocean ridge basalt (MORB), although they locally display small Ta, Ti and Nb negative anomalies, indicating a small subduction influence. This behavior changes to an island arc tholeiites (IAT) signature in the upper units of fine-grained amphibolites and amphibole rich-amphibolites, characterized by progressive depletion in the incompatible elements and more pronounced negative Ta and Nb anomalies, as well as common Ti and Zr negative anomalies. Tectono-magmatic variation diagrams and chondrite-normalized REE and primitive mantle normalized patterns suggest a back-arc to intra-oceanic island arc tectonic regime for the eruption of these rocks. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Accordingly, the Trincheira Complex is a record of oceanic crust relics obducted during the collision of the Amazon craton and the Paraguá block during the Middle Mesoproterozoic. Thus, the recognition of the Trincheira ophiolite and suture significantly changes views on the evolution of the southern margin of the Amazon craton, and how it can influence the global tectonics and the reconstruction of the continents.

(Table 2) Geochemistry of ore minerals and magnetization properties at the DSDP Leg 73 Holes

The samples investigated in this study come from DSDP Leg 73 Drill Holes 519A, 522B, and 524, all of which are in the South Atlantic. A general petrographic description of the basalts is given by Carman et al. (1984; doi:10.2973/dsdp.proc.73.120.1984).