The Journal of Geology

doi: 10.1086/629799pmid: N/A

In North Chile, Jurassic-Cretaceous arc magmatism is represented by several narrow belts of plutons and two separate volcanic sequences. New $$^{40}Ar/^{39}Ar$$ mineral ages confirm five distinct episodes of magma emplacement as plutonic complexes (c. 202-188 Ma, c. 160-153 Ma, c. 142-138 Ma, c. 130-127 Ma, and c. 106-103 Ma). Conjugate dike sets were emplaced immediately before periods of plutonism, and each distinct episode of magmatism prior to c. 127 Ma was located successively eastward, inboard from the subduction boundary, features interpreted to result from regional extension within the magmatic arc. Steeply dipping, ductile mylonitic shear zones were developed in wall-rocks along western contacts of Late Jurassic and Early Cretaceous plutons. Steeply-plunging mineral stretching lineations and a variety of kinematic indicators record an east-side-down extension. Along eastern faults of the Atacama Fault System steeply dipping, ductile mylonitic shear zones display subhorizontal mineral stretching lineations, and a variety of kinematic indicators record sinistral strike-slip displacement. Hornblende separated from mylonitic am-phibolite yields $$^{40}Ar/^{39}Ar$$ isotope correlation ages similar to those of adjacent plutons, indicating contemporaneous deformation and magma emplacement. Further, these ages suggest that a change from extensional dip-slip displacement to sinistral strike-slip displacement occurred at c. 126 Ma within the Atacama Fault System. Global plate reconstructions indicate that South America was essentially static in a mantle reference frame during the Jurassic-Early Cretaceous prior to c. 127 Ma, and the Andean plate boundary zone is interpreted to have been a retreating subduction boundary during this period of time. As a consequence, extensional deformation characterized the overriding plate, and magma emplacement appears to have been localized at ramps within a hinterland-propagating extensional duplex. Alternating episodes of predominantly volcanism or plutonism are interpreted to have reflected changes in the rate of subduction, whereas the change from a dip-slip extensional to a sinistral strike-slip tectonic regime is suggested to have resulted from a change in the convergence vector.

doi: 10.1086/629800pmid: N/A

Lapilli and unusually thick, high viscosity lavas extruded from Oldoinyo Lengai in June, 1993 are composed of crystal-rich natrocarbonatite containing a small proportion of porphyritic silicate spheroids. The silicate spheroids are of wollastonite nephelinite mineralogy, but they also contain natrocarbonatite mineral grains and aggregates (a) in glass inclusions within nepheline and pyroxene phenocrysts, and (b) within the fine-grained silicate groundmass of the spheroids. The chemical relationships of the natrocarbonatite component in the spheroids to the dominant silicate fraction are consistent with liquid immiscibility, as are the multiple stages of unmixing. These direct observations confirm the intimate relationship between natrocarbonatite and wollastonite nephelinite inferred both from the field relationships at Oldoinyo Lengai, and from low-PT experimentation. The plutonic equivalent (wollastonite ijolite) of the magma type found to be parental to natrocarbonatite at Oldoinyo Lengai does occur in other carbonatite complexes of differing ages and wide geographical distribution; hence it is possible that the generation of natrocarbonatite is not unique to Oldoinyo Lengai. Certain features in the matrix of the carbonatite flows (coarser grain size, breakdown of solid-solutions observed in earlier, rapidly chilled lavas) are attributed to the relatively slow cooling of these atypically thick flows.

doi: 10.1086/629802pmid: N/A

As magma rises through the lithosphere it may entrain wall rock debris. The entrainment process depends on the local hydrodynamic regime of the magma (e.g., velocity, temperature, bulk density), the extent of interaction of magma with groundwater, and the mechanical properties of the wall rocks. Wall rock entrainment results in local flaring of dikes and conduits, which in turn affects the hydrodynamics of magma ascent and eruption. We studied upper-crustal xenoliths erupted from small-volume basaltic volcanoes of the Lucero volcanic field (west-central New Mexico) in order to assess the relative importance of various entrainment mechanisms during a range of eruptive styles, including strongly hydrovolcanic, Strombolian, and effusive processes. Total xenolith volume fractions ranged from 0.3-0.9 in hydrovolcanic facies to $$<10^{-4}-10^{-2}$$ in Strombolian facies. The volcanoes erupted through a thick, well-characterized sequence of Paleozoic and lower Mesozoic sedimentary rocks, so that erupted xenoliths can be correlated with sedimentary units and hence depth ranges. The abundance of xenoliths from a given sub volcanic unit was divided by that unit's thickness to obtain an average entrainment rate (xenolith volume fraction derived per unit depth in the conduit). Shallow (<510 m) entrainment rates are very sensitive to the degree of hydrovolcanic activity. Deep entrainment is more sensitive to the mechanical properties of the wall rocks and, in the cases studied here, is thought to depend mainly on brittle failure related to offshoot dikes, pore pressure buildup, and thermal stresses. The entrainment rate can be used as a source term in multiphase numerical models of conduit flow. Based on our data, theoretical models of conduit flow and erosion are justified in neglecting the contribution of mass and momentum from entrained material during basaltic eruptions driven by magmatic volatiles, but not in eruptions driven by hydrovolcanic processes.

doi: 10.1086/629803pmid: N/A

The end-Paleozoic Pangea appears to have contained three continents that had grown in the Precambrian and remained intact until Mesozoic rifting: Ur, formed at ~3 Ga and accreted to most of East Antarctica in the middle Proterozoic to form East Gondwana; Arctica, an approximately 2.5-2 Ga continent that contained Archean terranes of the Canadian and Siberian shields and Greenland; and Atlantica, formed at ~2 Ga of cratons of ~2 Ga age that now occur in West Africa and eastern South America. Arctica grew at ~1.5 Ga by accretion of most of East Antarctica plus Baltica to form the continent of Nena. Collision of Nena, Ur, and Atlantica, plus minor plates, formed the supercontinent of Rodina at ~1 Ga. Rifting of Rodinia between 1 and 0.5 Ga formed three continents: East Gondwana; Atlantica (which became the nucleus for West Gondwana); and Laurasia (which contained North America, Greenland, Baltica, and Siberia). Gondwana formed at ~0.5 Ga by amalgamation of its eastern and western parts. Various plates accreted to Laurasia during the Paleozoic, and collision of Gondwana with Laurasia created Pangea at ~0.3 Ga.