The Journal of Geology

Hannisdal, Bjarte

doi: 10.1086/499569pmid: N/A

AbstractStratophenetic data document phenotypic changes in a fossil lineage and play a vital role in reconciling contemporary microevolution with longterm paleontological patterns. However, stratophenetic series represent multiscale geological and biological interactions, defying simple analysis and interpretation. A numerical model is presented that simulates stratophenetic series in shallow marine siliciclastic deposits. The model is driven by predictions of water depth, substrate properties, and sedimentation rate from a highresolution sedimentary basin fill model. Species abundance is modeled as a probability density peaked with respect to environmental preferences. The Price equation is used to model phenotypic evolution based on phenotypefitness covariance and drift. Preservation is a Poisson process controlled by population size, preservation probability, and sedimentation. Numerical experiments are used to investigate (1) the effects of sampling and depositional architecture on observed patterns and (2) the performance of various statistical tests in identifying evolutionary mode. As sample sizes decrease, the inaccuracy of sample mean values causes a stratophenetic pattern referred to as analytical stasis. Depositional architecture can cause nonrandom patterns through temporally irregular preservation, with the relative size and distribution of gaps being more important than the absolute size of gaps and overall completeness. For short series, statistical tests based on a randomwalk null hypothesis lose power and should be abandoned in favor of a multidimensional approach. A model of the data is needed that can account for confounding factors, and all available information on time and environment as well as phenotypic data should be incorporated and analyzed jointly, with a greater emphasis on quantifying uncertainty.

Bondre, N.R.; Hart, W.K.; Sheth, H.C.

doi: 10.1086/499568pmid: N/A

AbstractNumerous large, NESW to EWtrending mafic dikes outcrop around Sangamner in the western Deccan Volcanic Province. This area is part of a broader region postulated to be a shieldlike feature and a major eruption center. A combination of field, geochemical, and isotopic (Sr and Nd) characteristics is used here to understand the relationship of this dike swarm with the associated lava flows and their position in the established Deccan stratigraphy. Many dikes are compositionally similar to the Khandala and Poladpur formations belonging to the Lonavala and Wai subgroups, respectively, while one dike is similar to the Ambenali Formation. One dike has a composition distinct from all other dikes in this area as well as from most stratigraphic units, although there are many similarities in composition with the Bushe Formation as well as the Boyhare Member of the Khandala Formation. While several dikes are geochemically similar to specific flows/members within certain formations, their isotopic composition is often different, sometimes significantly so. This implies either that there is a greater range in isotopic composition for those members than previously realized or that magmas with different isotopic compositions underwent broadly similar petrogenetic evolution leading to similarities in elemental composition. NESWtrending Poladpur and/or Khandalalike dikes are concentrated in the central part of the area; these dikes appear to represent a vent system that could have fed southern, western, or eastern exposures of these younger formations. It is also possible, however, that some or many of the dikes along this system were simply latestage intrusions of magmas representing the younger formations.

Rajesh, V.J.; Yokoyama, K.; Santosh, M.; Arai, S.; Oh, C.W.; Kim, S.W.

doi: 10.1086/499571pmid: N/A

AbstractWe report here the occurrence of rare zirconiumbearing minerals,zirconolite (CaZrTi2O7) and baddeleyite (ZrO2),from an unusual ultramafic suite within the Achankovil Shear Zone (ACSZ) insouthern India. Zirconolite occurs as inclusions within spinel inphlogopitegraphite spinellite and shows characteristic development ofradial cracks. Baddeleyite is commonly observed as an included phase withinphlogopite from phlogopite dunite and graphitespinel glimmerite. Themineral also occurs less commonly within spinel and graphite fromgraphitespinel glimmerite. The composition of zirconolite ischaracterized by an enrichment of U and Th over rare earth elements. Baddeleyiteshows abundance of Zr with minor Hf, Ti, and U. The mode of occurrence alongwith the chemical composition of these minerals implies their formation asearlystage crystallization products from a silicaundersaturatedmelt that was enriched in carbonatitephile elements suchas Ca, Zr, Ti, and volatiles CO2 and H2O. We reportUPb chemical ages from the zirconolite that show a mean of\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\usepackage[OT2,OT1]{fontenc}\newcommand\cyr{\renewcommand\rmdefault{wncyr}\renewcommand\sfdefault{wncyss}\renewcommand\encodingdefault{OT2}\normalfont\selectfont}\DeclareTextFontCommand{\textcyr}{\cyr}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\landscape$$469\pm 11$$\end{document}Ma. Wecorrelate this age with the timing of emplacement and consolidation of theultramafic suite within ACSZ, and it is considerably younger than the lateNeoproterozoicCambrian ages reported from this zone. Our data suggestearly Ordovician carbonatitetype melts emplaced within ACSZ, which weidentify as a mantlerooted zone. We infer that deepseatedextension along the ACSZ probably triggered the generation of such melts,related to the extensional collapse of the orogen following the collisionalassembly of the Gondwana supercontinent.

Foden, John; Elburg, MarlinaA.; DoughertyPage, Jon; Burtt, Andrew

doi: 10.1086/499570pmid: N/A

AbstractThe Antarctic Ross and the Australian Delamerian orogenies are the consequence of stress transfer to the outboard trailing edge of the newly assembled Gondwana supercontinent. This tectonic reorganization occurred in the Early to Middle Cambrian on completion of PanAfrican deformation and subduction along the sutures between eastern and western Gondwanan continental fragments. Before this, Neoproterozoic to Early Cambrian rocks in eastern Australia were formed in a passive margin and record dispersion of Rodinia with consequent opening of the protoPacific. Our new UPb and RbSr geochronology shows that in the South Australian (Adelaide Fold Belt) domain of the Delamerian Orogen, contractional orogenesis commenced at \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\usepackage[OT2,OT1]{fontenc}\newcommand\cyr{\renewcommand\rmdefault{wncyr}\renewcommand\sfdefault{wncyss}\renewcommand\encodingdefault{OT2}\normalfont\selectfont}\DeclareTextFontCommand{\textcyr}{\cyr}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\landscape$$514\pm 3$$\end{document}Ma and persisted for 24 m.yr. until \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\usepackage[OT2,OT1]{fontenc}\newcommand\cyr{\renewcommand\rmdefault{wncyr}\renewcommand\sfdefault{wncyss}\renewcommand\encodingdefault{OT2}\normalfont\selectfont}\DeclareTextFontCommand{\textcyr}{\cyr}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\landscape$$490\pm 3$$\end{document}Ma, terminated by rapid uplift, cooling, and extension in association with posttectonic magmatism. Integration of new and published UPb and 40Ar39Ar geochronology from the entire RossDelamerian belt shows that although both the Delamerian and Ross have a synchronous late magmatic and terminal cooling history, the Ross commenced its convergent orogenic history at 540 Ma. This was 25 m.yr. before Delamerian deformation began. During the Early Cambrian, eastern Australia was still in a state of extension (or transtension), with opening of the Kanmantoo Basin and associated anorogenic, largely mafic magmatism. This basin received sediment from the already exposed Ross Orogen to the south. The simultaneous first occurrence of strain fabrics and subductionrelated magmatism (including boninite, granite, and andesite lavas) at 514 Ma in New Zealand, Victoria, South Australia, New South Wales, and Tasmania implies that the Delamerian Orogeny was driven by ridgepush forces transmitted on the initiation of westwarddipping subduction. Subsequent eastward slab rollback at 490 Ma may have occurred when the new slab had reached the transition zone at 650km depth, resulting in upper plate extension and anorogenic Basin and Rangestyle magmatism in South Australia and Tasmania (Mount Read belt). The delayed onset of subduction in the Australian sector of the margin implies that westward motion of the Australian portion of eastern Gondwana continued to be accommodated during the late Early Cambrian by subduction or deformation along either the Mozambique Suture or at the northern end of the South Prince Charles MountainsPrydz Bay suture.

Offler, R.; Shaw, S.

doi: 10.1086/499572pmid: N/A

AbstractIn the southern New England Fold Belt, the PeelManning Fault System(PMFS) is a major structural element characterized by tectonic blocks in aserpentinite mlange. We document a large (>1 km), intensely deformedblock of noncumulate and cumulate hornblende gabbro from the PMFS. Cumulatelayering is preserved in some outcrops, and relicts of magmatic plagioclase,magnesiohornblende, and minor clinopyroxene are recognizablemicroscopically despite the gabbro having undergone amphibolite facies andsubsequently prehnitepumpellyite facies metamorphism. The composition ofthe magmatic clinopyroxenes and amphiboles indicates lowpressurecrystallization, suggesting formation in a shallow crustal reservoir.Zr/TiO2 ratios, chondritenormalized rare earth elements(\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\usepackage[OT2,OT1]{fontenc}\newcommand\cyr{\renewcommand\rmdefault{wncyr}\renewcommand\sfdefault{wncyss}\renewcommand\encodingdefault{OT2}\normalfont\selectfont}\DeclareTextFontCommand{\textcyr}{\cyr}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\landscape$$[ \mathrm{La}\,/ \mathrm{Yb}\,] _{\mathrm{N}\,}=6.4{\mbox{--}} 2.7$$\end{document}),and rock/MORB (midocean ridge basalt) patterns indicate that the gabbroshave a calcalkaline, basaltic magmatic affinity and a compositiontransitional between oceanic and continental arc rocks. The mafic rocksintermingled with and intruding the gabbro have a calcalkaline arc totholeiitic backarc basin affinity (\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\usepackage[OT2,OT1]{fontenc}\newcommand\cyr{\renewcommand\rmdefault{wncyr}\renewcommand\sfdefault{wncyss}\renewcommand\encodingdefault{OT2}\normalfont\selectfont}\DeclareTextFontCommand{\textcyr}{\cyr}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\landscape$$[ \mathrm{La}\,/ \mathrm{Yb}\,] _{\mathrm{N}\,}=2.8{\mbox{--}} 1.2$$\end{document}).Y/15La/10Nb/8 plots confirm the arc affinity of these rocks.Peak metamorphic assemblages formed at temperatures of 590C andpressures of 550 MPa, suggesting that a geothermal gradient of 29C/kmoperated during deformation. Singlezircon UPb analyses from tworock samples by laser ablation quadrupole inductively coupled plasma massspectrometry (ICPMS) yielded a combined age of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\usepackage[OT2,OT1]{fontenc}\newcommand\cyr{\renewcommand\rmdefault{wncyr}\renewcommand\sfdefault{wncyss}\renewcommand\encodingdefault{OT2}\normalfont\selectfont}\DeclareTextFontCommand{\textcyr}{\cyr}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\landscape$$444.7\pm 2.4$$\end{document}Ma(2). Further, LuHf TDM model agesobtained from single zircons from the same samples analyzed by laser ablationmulticollector ICPMS were calculated as \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\usepackage[OT2,OT1]{fontenc}\newcommand\cyr{\renewcommand\rmdefault{wncyr}\renewcommand\sfdefault{wncyss}\renewcommand\encodingdefault{OT2}\normalfont\selectfont}\DeclareTextFontCommand{\textcyr}{\cyr}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\landscape$$447\pm 25$$\end{document}Ma(2). The similarity of zircon crystallization ages (UPb) andTDM model ages (LuHf) suggests little ifany crustal residence between the time of partial melting of depleted mantle toproduce the gabbro magma and the time of zircon crystallization. This studyprovides evidence for Late Ordovician arc magmatism and links all the earlyPaleozoic arcrelated events recognized previously.

Barth, AndrewP.; Wooden, JosephL.

doi: 10.1086/499573pmid: N/A

AbstractInitiation of the Cordilleran magmatic arc in the southwestern United States is marked by intrusion of granitic plutons, predominantly composed of alkalicalcic Fe and Srenriched quartz monzodiorite and monzonite, that intruded Paleoproterozoic basement and its Paleozoic cratonalmiogeoclinal cover. Three intrusive suites, recognized on the basis of differences in high field strength element and large ion lithophile element abundances, contain texturally complex but chronologically distinctive zircons. These zircons record heterogeneous but geochemically discrete mafic crustal magma sources, discrete PermoTriassic intrusion ages, and a prolonged postemplacement thermal history within the longlived Cordilleran arc, leading to episodic loss of radiogenic Pb. Distinctive lower crustal magma sources reflect lateral heterogeneity within the composite lithosphere of the Proterozoic craton. Limited interaction between derived magmas and middle and upper crustal rocks probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin magmatic arc.

Adams, AaronJ.; Christiansen, EricH.; Kowallis, BartJ.; CarranzaCastaeda, Oscar; Miller, WadeE.

doi: 10.1086/499633pmid: N/A

AbstractThe San Miguel de Allende graben, Guanajuato, Mexico, contains numerous rhyolitic volcanic ash beds. Electron microprobe and xray fluorescence analyses of glass shards from 14 localities, combined with mineralogic, stratigraphic, radiometric, and paleomagnetic data, allow us to correlate the ash beds (and the intervening sedimentary strata and fossils), understand the timing of volcanism, date the age of extension, and better understand the tectonic and volcanic evolution of central Mexico. Our analyses reveal that at least six separate eruptions of rhyolitic ash occurred during the Late Miocene and Pliocene (53 Ma) while the San Miguel Allende basin was subsiding. The fallout ash beds can be distinguished by phenocryst mineralogy, inferred eruption temperatures, and differences in major and trace element compositions. Two magma series (medium K and high K) are represented in the rhyolitic tephra deposits. The highK series is marginally peralkaline, reduced (indicated by high Fe/Mg ratios), dry (paucity of hydrous silicates), hot (860C), and enriched in many incompatible trace elements including Nb, Y, and Zr. They have the characteristics of anorogenic rhyolites formed in rift and hotspot settings. In contrast, the rhyolites of the mediumK series are similar to those erupted in subductionrelated volcanic arcs. They are calcic to calcalkalic with low Fe/Mg ratios and have hydrous phenocrysts, suggesting the water fugacity was also high. Eruption temperatures were less than 860C. However, these mediumK rhyolites have high Sr/Y ratios and were probably derived by differentiation of adakitic parents. We suggest the contrasting rhyolite series are the result of differentiation of two different parental magma types. During the late Miocene to Pliocene slab breakoff, hot asthenosphere rose through a break in a subducting slab of oceanic lithosphere. Parents of the mediumK series (adakitic magmas) formed as hot rising mantle caused the edge of the torn plate to melt. HighK parental magmas were formed by decompression melting as asthenospheric mantle rose through the gap in the slab.