Kenneth A. Foland

Magmatism along the southeast margin of the Yangtze block: Precambrian collision of the Yangtze and Cathysia blocks of China

The time and the environment of formation of igneous rocks along the southern margin of the Yangtze continental block in southern Anhui Province and northeastern Jiangxi Province, China, are discussed. Sm-Nd dating for samples from two ophiolite suites give ages of about 1 Ga. Late Proterozoic granodiorites in the study area have ages of about 0.9 Ga and have normal intrusive relations against Banxi Group metamorphic country rocks. Trace element abundances and initial Nd isotope ratios for ophiolite and Proterozoic granodiorites are consistent with formation in an island-arc environment. In contrast, Sr and Nd isotopes for Mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the Yangtze block. These relations do not support a recently proposed Mesozoic overthrust tectonic model for southeast China, but rather favor a late Precambrian collision history for the Yangtze and Cathysia blocks.

Insights from the Talysh of Azerbaijan into the Paleogene evolution of the South Caspian region

The age and mode of formation of the South Caspian Basin are disputed. An ~10-km-thick, predominantly middle Eocene clastic and volcanic succession is exposed in the Talysh mountains of Azerbaijan at its western margin. Here, high-K alkali basalts pass laterally to the east and southeast into volcanogenic sandstone-dominated turbidity current and debris-flow deposits. These southeasterly directed depositional systems accumulated in water depths generally greater than 200 m and fed directly into the western South Caspian Basin. New Ar-Ar ages cluster around 39 Ma, with an upper, 1400-m-thick volcanic interval being deposited in 2.2 ± 0.2 m.y. We interpret that this rapid deposition and magmatism records a major back-arc extensional/transtensional event in the Talysh, north of the north-dipping Neotethyan subduction zone. This event is recognized across much of southwest Asia and may indicate a period of significant basin formation within the adjacent South Caspian Basin. A transition into Upper Eocene–Lower Oligocene strata, dominated by fine-grained turbidity current and hemipelagic sediments with slope instability features, is interpreted to mark the end of rifting and volcanism in the Talysh and the start of the Arabia-Eurasia collision. Overlying Oligocene coarse clastic rocks are interpreted as the erosional products of localized topography created by the further propagation of compressional deformation into the Talysh region.

A short interval of Jurassic continental flood basalt volcanism in Antarctica as demonstrated by 40Ar39Ar geochronology

A continental flood basalt province, the Ferrar Group (Kirkpatrick Basalt and Ferrar Dolerite), crops out along 3000 km of the Transantarctic Mountains in Antarctica and is temporally related to the break-up of Gondwanaland. Although a wide range of dates, between 90 and 193 Ma, have been published for the Kirkpatrick Basalt, it is now recognized that the young dates reflect non-ideal behavior of Ar in the matrix. In order to refine the geochronology, feldspar separates have been analyzed by the 40Ar39Ar incremental heating method. The main objectives are to constrain the duration of extrusive activity and the timing of volcanism along the outcrop belt. Basalt samples have been studied from the three principal outcrop areas, yielding the following apparent ages: central Transantarctic Mountains 176.8 ± 0.5 Ma; south Victoria Land 176.4 ± 0.4 Ma; north Victoria Land 176.6 ± 0.7 Ma. Ages from different stratigraphic levels within each area and from the three different areas are not analytically distinct. The data imply that the eruptive activity which produced the Kirkpatrick Basalt occurred within a short interval of less than about 1 m.y. at 176.6 ± 1.8 Ma, over an area which included more than 1200 km of the Transantarctic Mountains. The Jurassic volcanism in Antarctica represents a short episode of magmatism, comparable in duration with other well dated continental flood basalt provinces. The linearly extensive outcrop of the Ferrar Province and the rapid eruption of the lavas suggests that lithospheric stretching exerted a major control on magmatism. The poorly constrained age of the Bajocian-Bathonian boundary makes the previously suggested connection between Ferrar volcanism and an extinction event at that boundary uncertain.

Magma sources for Mesozoic anorogenic granites of the White Mountain magma series, New England, USA

The magma sources for granitic intrusions related to the Mesozoic White Mountain magma series in northern New England, USA, are addressed relying principally upon Nd isotopes. Many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts. Sm−Nd and Rb−Sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 Ma. The possibility of crustal sources is further examined with analyses of representative older crust including Paleozoic granitoids which serve as probes of the lower crust in the region. Multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations (<∼1 in εNd). Overall, Mesozoic granite plutons range in εNd [T] from +4.2 to -2.3, with most +2 to 0, and in initial 87Sr/86Sr from 0.7031 to ∼0.709. The isotopic variations are roughly inversely correlated but are not obviously related to geologic, geographic, or age differences. Older igneous and metamorphic crust of the region has much lower Nd isotope ratios with the most radiogenic Paleozoic granitoid at εNd [180 Ma] of -2.8. These data suggest mid-Proterozoic separation of the crust in central northern New England. Moreover, the bulk of the Mesozoic granites cannot be explained as crustal melts but must have large mantle components. The ranges of Nd and Sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources. Crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation. The results indicate production of anorogenic granites from mantle-derived mafic magmas.

Long-distance transport of magmas in the Jurassic Ferrar Large Igneous Province, Antarctica

Abstract The youngest preserved lava flows of the Jurassic Kirkpatrick Basalt of the Ferrar Large Igneous Province are unusually distinctive. These flows, recognized in three geographic areas spread over 1600 km, have identical stratigraphic positions and geochemical characteristics. The lavas have an evolved, Fe-rich tholeiitic composition with lithospheric trace element ratios and enriched 87Sr/86Sr and 143Nd/144Nd isotope ratios. Except for the highly mobile elements, the major, trace and isotopic compositions of lavas from all these localities lie within, or near, analytical precision of each other, and are distinct from other Ferrar rocks. Moreover, the ages of the flows at all localities are indistinguishable. The unique characteristics of these capping lavas suggest that they were derived from a single batch of magma. Magma dispersal from a single reservoir through dike swarms at middle to upper crustal levels is considered the most probable mechanism for large-scale transport that extended for more than 3000 km.

40Ar/39Ar geochronology of Ferrar Dolerite sills from the Transantarctic Mountains, Antarctica: Implications for the age and origin of the Ferrar magmatic province

The Ferrar Dolerite constitutes the hypabyssal phase of the tholeiitic Ferrar Group of Antarctica. Sills with compositions representing most of the range of geochemical variation of the Ferrar Dolerite, and separated by distances of as much as 1400 km, have been analyzed by the 40 Ar/ 39 Ar method on feldspar and biotite separates. The 40 Ar/ 39 Ar ages for five individual sills range from 176.2 to 177.2 Ma and show no significant difference. These ages reflect crystallization at 176.7 ± 1.8 Ma (where the uncertainty includes provision for systematic uncertainty in the age of the neutron-fluence monitor calibrated relative to MMhb-1 at 513.5 Ma). Combining data from these sills with previous determinations on coeval lavas and underlying pyroclastic units indicates an age of 176.6 ± 1.8 Ma for the Ferrar tholeiitic rocks as a whole. The duration of magmatic activity was less than approximately 1 m.y. By extension, other rocks in the Ferrar magmatic province, which occur from southeastern Australia, along the Transantarctic Mountains to the Theron Mountains, are inferred to have this age. The short duration of magmatic activity as well as the consistent pattern of geochemical variation and distinctiveness of the Ferrar rocks suggest that magmas were transported laterally by an extensive dike swarm which is inferred to have originated in the Weddell Sea sector of the province.

Potassium-argon dating of fine-grained basalts with massive Ar loss: Application of the 40Ar39Ar technique to plagioclase and glass from the Kirkpatrick Basalt, Antarctica

40Ar39Ar incremental-heating measurements are reported for separated plagioclase and matrix fractions and a glass sample of the Kirkpatrick Basalt from Victoria Land, Antarctica. These results are used to address: the limitations of whole-rock analyses of glassy or very fine-grained basalts; the use of plagioclase for determining ages of such rocks; the Ar behavior of matrix, glass and feldspar components; the use of K/Cl ratios in interpretation of step-heating results; and the age of the Kirkpatrick Basalt. The precise age of the Kirkpatrick Basalt has been a long standing problem; the wide range of reported K-Ar dates has been attributed to loss of Ar. Despite massive Ar loss from the matrix, plagioclase is shown to yield good 40Ar39Ar plateaus. Plateaus are observed for increments above 700°C and are defined not only by consistent apparent ages but also appropriately low K/Ca and high K/Cl ratios. Cl relations help identify Ar released from small amounts of matrix which can seriously contaminate a plagioclase separate. An age of 176.8 ± 1.8 Ma defines the time of formation of the youngest lava at Mt. Frustum in north Victoria Land. A glass sample from south Victoria Land gives the same age, indicating that terrestrial basaltic glasses as old as Jurassic do not inherently lose Ar. The step-heating profile of glass with very fine crystals shows discordance which is an artifact of Ar recoil during irradiation.

New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming province with Laurentia

A question regarding the 1900–1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40 Ar/ 39 Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes (F 1 ) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest–trending upright F 2 folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40 Ar/ 39 Ar hornblende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is ∼50–60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810–1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780–1740 Ma interval of island-arc accretion along the southern margin of the growing craton.

Tectonomagmatic Evolution of Bimodal Plutons in the Central Anatolian Crystalline Complex, Turkey

The NW‐trending Agaçören Intrusive Suite (AIS) on the east side of the Salt Lake (Tuz Gölü), Turkey, is part of a curvilinear volcanoplutonic complex along the western edge of the central Anatolian crystalline complex (CACC). Granitoids constitute the predominant lithological group within the AIS and range in composition from monzonite through granite to alkali feldspar granite. Gabbroic rocks occur as irregular intrusive bodies with sinusoidal, irregular contacts with the granitoid plutons and vary from dioritic compositions at the contacts to cumulate amphibole gabbros in their center. These gabbroid to granitoid rocks are all subalkaline and display tholeiitic to calcalkaline affinities, respectively. Modeling of the major element and trace element chemistry of the granitoids to gabbroids, together with their field relations, suggest that these rocks formed synchronously from a melt source that involved both mantle and crustal components. Mafic magmas were likely derived from a metasomatized upper mantle source above a subduction zone and were injected into crustal‐level felsic magma chamber(s) in which incomplete mixing, commingling, and crystal fractionation processes produced the coeval granitoid to gabbroid plutons. New 40Ar/39Ar ages 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