Samuel B. Mukasa

Antarctica-New Zealand rifting and Marie Byrd Land lithospheric magmatism linked to ridge subduction and mantle plume activity

Mid-Cretaceous igneous rocks of central Marie Byrd Land, Antarctica record a rapid change from subduction-related to rift-related magmatism. This correlates with the final stages of subduction of the Phoenix plate and the subsequent rifting of New Zealand from West Antarctica, prior to the opening of the Southern Ocean. Rift magmatism produced diverse A-type granitoids and mafic intrusive rocks of continental flood-basalt affinity that were derived ultimately from lithospheric mantle sources. Rifting was caused by changes in plate boundary forces; however, mantle plume activity may have begun in mid-Cretaceous time, triggering melting of the lithosphere and controlling the locus of rifting.

Marie Byrd Land, West Antarctica: Evolution of Gondwana's Pacific margin constrained by zircon U-Pb geochronology and feldspar common-Pb isotopic compositions

The Paleozoic and Mesozoic development and subsequent fragmentation of Gondwanas Pacific margin are recorded in igneous and metamorphic rocks that crop out in Marie Byrd Land, West Antarctica, recognized on geologic and paleomagnetic grounds to compose a discrete crustal block. Widespread metaluminous granitoids dated by the zircon U-Pb method as middle to late Paleozoic show that convergence-related magmatism dominated the early evolution of this margin. Dates for granodiorites, monzogranites, and granites from the Ruppert and Hobbs coasts of western Marie Byrd Land reveal a prolonged period of subduction-related calc-alkaline magmatism between at least 320 ± 3 Ma (age of the oldest granodiorite dated) and 110 ± 1 Ma (the age of the youngest I-type granitoid in the area). The latter, known as the Mount Prince granite, is intruded by swarms of mafic and intermediate dikes believed to record the onset of rifting that led to separation of the New Zealand microcontinent. The dikes have been dated by zircon U-Pb as 101 ± 1 Ma. Thus, the regime along the Ruppert and Hobbs coasts had shifted from subduction-related to rift-related magmatism within an ∼9 m.y. period. In the Kohler Range and the Pine Island Bay areas of eastern Marie Byrd Land, the calc-alkaline magmatism did not terminate until 96 ± 1 Ma, based on U-Pb dating of zircons from one granitoid sample, or 94 ± 3 Ma based on zircons from another. This evidence requires that subduction shut off from west to east, as suggested previously on the basis of geophysical models. No continental separation occurred to the east of Marie Byrd Land. The margins of the Thurston Island and Antarctic Peninsula crustal blocks went directly from convergent to inactive, except at the northernmost tip of the peninsula, where the South Shetlands Island block is actively separating. With their zircon U-Pb ages clustering around 100 ± 2 Ma, dike-free anorogenic syenites and quartz syenites along the Ruppert and Hobbs coasts show that the transition to extensional magmatism was rapid in the west. This is also reflected by the fact that from the onset of rifting at 101 ± 1 Ma to formation of oceanic crust between Marie Byrd Land and greater New Zealand (Campbell Plateau, Chatham Rise, North Island, and South Island) prior to chron 33o ca. 81 Ma required only 20 m.y. For comparison, this is only two-thirds of the ∼30 m.y. it took for the Central Atlantic to open after initial rift-related magmatism. The swiftness of the separation between Marie Byrd Land and greater New Zealand demonstrated by our data is consistent with ridge-trench interaction rather than a mantle plume as the primary cause of the breakup, as is the west to east diachroneity in the cessation of subduction. Exposures of host rocks to the erosion-resistant plutons are scarce in mostly snow- and ice-covered Marie Byrd Land. The occurrence in the zircons of widely separated granitoids of discordant U-Pb patterns we attribute to inheritance (the best-constrained upper concordia intercepts are as high as 1576 ± 55 Ma). This suggests either that stretched Precambrian basement underlies most of Marie Byrd Land, or that clastic sedimentary sequences with Precambrian detrital zircons underlie much of the margin.

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite 40Ar39Ar age constraints on tectonic evolution of Southwestern Gondwanaland

Abstract Zircon U-Pb and muscovite 40 Ar 39 Ar isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic “back stop” of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the 40 Ar 39 Ar age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.

Zircon UPb and hornblende 40Ar39Ar ages for the Dufek layered mafic intrusion, Antarctica: Implications for the age of the Ferrar large igneous province

Abstract New high-precision zircon UPb ages for the Dufek intrusion establish the crystallization age for the only layered mafic intrusion (sensu stricto) known in the Ferrar igneous province, Antarctica, one of the largest igneous provinces in the world. Three concordant zircon fractions from the capping Lexington Granophyre yield an age of 183.9 ± 0.3 Ma (2σ), which we interpret as the crystallization age for the Dufek intrusion. Concordant zircon fractions from a silicic dike crosscutting the upper layered gabbroic sequence yield a slightly younger crystallization age of 182.7 ± 0.4 Ma (2σ) . For both samples, agreement within errors between the two UPb and 207 pb∗/ 206 Pb∗ ages for three fractions each with a unique set of physical and chemical characteristics demonstrates that inheritance of older zircon components from the country rocks was imperceptible. It also suggests that assimilation or anatexis of the lower Paleozoic country rocks was not the dominant mechanism in the formation of the granophyre. These new UPb dates agree, within analytical errors, with recent zircon and baddeleyite UPb ages for dolerite sills in both the Ferrar and Karoo igneous provinces, which demonstrates the vastness of the rift axis along which simultaneous magmatic activity occurred. Contemporaneity between the Dufek intrusion and other Ferrar Group rocks supports the possibility that this pluton acted as a conduit for magmas of the widely distributed Ferrar sills and flows. Hornblende 40 Ar 39 Ar ages for the Lexington Granophyre (175 ± 2 Ma) barely overlap with those for a silicic dike (179 ± 2 Ma) in the layered gabbro section just below the granophyre. The younger of the two hornblende ages may be explained by degassing of inclusions or by protracted hydrothermal alteration preventing the ArAr geochronometer form becoming a closed-system in the higher level granophyre until after it had done so in the more deeply positioned silicic dike. The 179 ± 2 Ma age of the Dufek silicic dike is interpreted to represent cooling through the Ar retention temperature for hornblende of about 500°C. Combining this cooling age with the UPb crystallization age of 182.7 ± 0.4 Ma yields a cooling rate for the dike of approximately 100°C/m.y. Inasmuch as this rate is not rapid, it can be inferred that the bulk of the Dufek intrusion was still fairly hot when the dated silicic dike was emplaced.

Quartz exsolution in clinopyroxene is not proof of ultrahigh pressures: Evidence from eclogites from the Eastern Blue Ridge, Southern Appalachians, U.S.A.

Abstract Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahighpressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca.Eskola component (Ca0.5⃞0.5AlSi2O6, CaEs) that is presumed to be stable only at UHP conditions. Eclogite from the Eastern Blue Ridge, North Carolina, contains clinopyroxene (Jd20CaTs5Ac5CaEs0Di65Hd5) with oriented needles of quartz and calcic amphibole that appear to have exsolved together. The quartz + amphibole intergrowths are surrounded by 1.5 µm haloes of neoformed pyroxene (Jd10CaTs10Ac5CaEs0Di70Hd5). The modes of quartz, amphibole, and clinopyroxene haloes were determined using BSE images, and reintegrated with the host clinopyroxene. Viewing the quartz and amphibole needles down the c-axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. Clinopyroxene with CaEs solid solution has been repeatedly synthesized at UHP conditions. However, examination of the phase equilibria usually cited as evidence for CaEs stability at conditions of ≥25 kbar shows that clinopyroxene with 10 mol% CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite = jadeite + quartz barometer. Exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the Blue Ridge eclogite that lacks coesite or other evidence for UHP metamorphism. The presence of a diluted (5.10%) CaEs component in clinopyroxene does not require UHP conditions.

A multielement geochronologic study of the Great Dyke, Zimbabwe: Significance of the robust and reset ages

Abstract New Sm–Nd, U–Pb, and Pb–Pb age determinations indicate that the Great Dyke of Zimbabwe, an elongate intrusion of mafic and ultramafic rocks some 550 km long and between 3 and 10 km wide, is over 100 Ma older than previously believed based on Rb–Sr ages. The intrusion was emplaced as a series of subchambers with similar stratigraphy, comprising a lower ultramafic sequence with cyclic layering of dunite or harzburgite grading upwards into bronzitite, the top sections of which include Pt-enriched sulfide zones, and an upper mafic sequence of pyroxenites capped by olivine gabbro and gabbronorite. The Sm–Nd method has yielded a combined mineral/whole-rock isochron of 2586±16 Ma and e Nd ( t ) of +1.1 for samples from the Darwendale, Sebakwe, and Wedza Subchambers as well as the satellite East Dyke. This isochron age is in excellent agreement with the U–Pb age for three concordant rutile fractions extracted from a feldspathic pyroxenite of the Selukwe Subchamber with an error-weighted mean at 2587±8 Ma. Two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant, and although not well constrained, suggest Pb loss from the zircons at ca. 830 Ma. This may be related to the onset of the widespread and diachronous Pan-African tectonothermal event in southern Africa. Whole–rock samples and clinopyroxene and plagioclase separates from a Darwendale Subchamber drill core yielded a 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb isochron age of 2596±14 Ma, which is in agreement with the Sm–Nd isochron and the rutile U–Pb crystallization age. This new age information shows that emplacement of the Great Dyke and its satellite dikes closely followed the amalgamation of the Kaapvaal and Zimbabwe Cratons, and was contemporaneous with emplacement of the youngest of the trondhjemite–tonalite–granodiorite granitoid suite in the Zimbabwe Craton. Assuming that amalgamation of the Kaapvaal and Zimbabwe Cratons was largely by NNW-directed convergence, it follows that the source of the Great Dyke was asthenospheric mantle hydrated and enriched in incompatible elements by subduction processes. Isochrons of 206 Pb/ 204 Pb vs. 238 U/ 204 Pb and 207 Pb/ 204 Pb vs. 235 U/ 204 Pb yield ages with large errors, but well constrained initial Pb ratios ( 206 Pb/ 204 Pb = 14.15±0.30 and 207 Pb/ 204 Pb = 15.04±0.06). Assuming a two-stage model for common lead evolution, this result yields a μ value of 9.5. Along with the calculated initial Sr and Nd isotopic compositions, these data are consistent with derivation of the Great Dyke magmas by large volume melting of a mantle that has been hydrated and enriched by subduction. While a small amount of crustal contamination of magma derived from depleted mantle could produce the composition of the Great Dyke, the uniformity of initial ratios between subchambers supports the notion of enrichment in incompatible elements being an intrinsic characteristic of the mantle source.

Growth of subcontinental lithosphere: Evidence from repeated dike injections in the Balmuccia lherzolite massif, Italian Alps

Abstract The Balmuccia alpine lherzolite massif is a fragment of subcontinental lithospheric mantle emplaced into the lower crust 251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists largely of lherzolite, with subordinate harzburgite and dunite, and an array of dike rocks formed in the mantle before crustal emplacement. Dike rocks include websterite and bronzitite of the Cr-diopside suite, spinel clinopyroxenite and spinel-poor websterite of the Al-augite suite, gabbro and gabbronorite of the late gabbro suite, and hornblendite of the hydrous vein suite. The dike rocks display consistent intrusive relationships with one another, such that Cr-diopside suite dikes are always older than dikes and veins of the Al-augite suite, followed by dikes of the late gabbro suite and veins of the hydrous vein suite. Phlogopite (phl) veinlets that formed during interaction with the adjacent crust are the youngest event. There are at least three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a polybaric fractionation series from spinel clinopyroxenite to websterite and feldspathic websterite, which crystallized from aluminous alkaline magmas at relatively high pressures. The late gabbro suite of dikes intruded at lower pressures, where plagioclase saturation occurred before significant mafic phase fractionation. Hornblendite veins have distinct compositional and isotopic characteristics, which show that they are not related to either the Al-augite suite or to the late gabbro dike suite. Cr-diopside suite dikes have Nd and Sr isotopic compositions similar to those of the host lherzolite and within the range of compositions defined by ocean–island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic compositions similar to those of Cr-diopside suite lherzolite and websterite. The late gabbro dikes have Nd and Sr isotopic compositions similar to mid-ocean ridge basalt (MORB) asthenosphere. Lead isotopic compositions for all of the samples fall in the present-day MORB field on the 208 Pb / 204 Pb vs. 206 Pb / 204 Pb diagram but are displaced above this field on the 207 Pb / 204 Pb vs. 206 Pb / 204 Pb diagram. There is overlap in the data between the Cr-diopside suite and the Al-augite and hydrous vein suites, with the exception that the Cr-diopside websterite dikes have more radiogenic Pb than any of the other samples. In Pb–Pb space as well, the late gabbro suite has the least radiogenic isotopic compositions, reflecting a change in magma source region during uplift. These data show that tectonic thinning of subcontinental lithospheric mantle during extension caused a change in the source regions of mantle-derived magmas from an ocean island basalt (OIB)-like lithosphere to the underlying MORB asthenosphere. They also demonstrate that the upper mantle acquires its heterogeneous isotopic character through several different processes, including in situ radiogenic growth, addition of asthenospheric melts, dike-wall rock ionic exchange, redistribution of the lithospheric dike and vein materials by melting, and in the late stages of emplacement, assimilation of crustal materials.

NdSrPb isotopic, and major- and trace-element geochemistry of Cenozoic lavas from the Khorat Plateau, Thailand: sources and petrogenesis

Abstract Basaltic rocks from Khorat Plateau, Thailand, dated at 0.9 Ma, coincide in time with the lithospheric extension of continental southeast Asia that began in the mid-Cenozoic. Dominated by alkali-olivine basalt and hawaiite compositions, they are generally alkalic and show specific petrologic and geochemical variations. Their trace-element and isotopic compositions are generally similar to those of ocean island basalts, and define two distinct groups. The group-I rocks have moderately depleted and relatively homogeneous isotopic ratios with 143 Nd 144 Nd = 0.51287−0.51296 87 Sr 86 Sr = 0.70354−0.70388 and Pb-isotopic ratios that are fairly nonradiogentic ( 206 Pb 204 Pb = 18.23−18.32 , 207 Pb 204 Pb = 15.47−15.53 and 208 Pb 204 Pb = 38.16−38.27 ). The group-II rocks show an enriched isotopic signature with 143 Nd 144 Nd = 0.51266−0.51281 , 87 Sr 86 Sr = 0.70486−0.70585 and more radiogenic Pb-isotopic ratios ( 206 Pb 204 Pb = 1849−1865 , 207 Pb 204 Pb = 15.54−15.60 and 208 Pb 204 Pb = 38.48−38.84 ). These isotopic data display linear trends on the 143 Nd 144 Nd and 206 Pb 204 Pb vs. 87 Sr 86 Sr diagrams, with group-I rocks clustering near the less depleted end of the field for Indian Ocean MORB and group-II rocks extending toward the EM2 (i.e. enriched mantle of type 2) end-member. Thus two source domains, one with a moderately depleted Indian Ocean MORB-like isotopic signature and the other with a EM2-like character, are thought to have been involved in the generation of these lavas, with the former originating from the asthenospheric mantle, and the latter likely from the lithospheric mantle. We suggest that the primary melts of group-I rocks formed by decompressional polybaric partial melting of asthenospheric materials similar in their isotopic compositions to the source of less depleted Indian Ocean MORB. This was followed by some fractional crystallization, chiefly of olivine, but with lottle contamination by the continental lithospheric materials en route to the surface. However, variations in the incompatible major- and trace-element concentrations of the group-I rocks are mainly due to the different pressures of melting (i.e. different depths) and different degrees of partial melting of source materials in the asthenosphere. The group-II rocks which show an enriched isotopic signature, on the other hand, are interpreted to be products of mixing between materials with highly radiogenic Sr and Pb, and nonradiogenic Nd, most likely aged frozen melts in the lithospheric mantle and young asthenospheric melts similar in their isotopic character to the moderately depleted Indian Ocean MORB. It is likely that this asthenospheric source is prevalent beneath continental southeast Asia.

Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridotite and anhydrous and hydrous pyroxenite, some cut by igneous-textured pyroxenite-gabbro veins and dikes and some by veins of amphibole ± plagioclase. Igneous-textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma-mantle wall rock reactions, episodic intrusion and infiltration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (eNd = −5.7 to +3.4; 87Sr/86Sr = 0.7051 – 0.7073; 206Pb/204Pb = 19.045 – 19.195) and the igneous-textured xenoliths (eNd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 – 0.7036 with one carbonate-affected outlier at 0.7054; and 206Pb/204Pb = 18.751 – 19.068), so that they cannot be related. The igneous-textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have eNd of +5.1 to +9.3; 87Sr/86Sr of 0.7028 – 0.7050, and 206Pb/204Pb of 18.685 – 21.050. The igneous-textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous-textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm-Nd two-mineral “isochrons” yield apparent ages for petrographically identical rocks believed to be coeval ranging from ∼0 to 113±26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr-diopside group isotopic compositions do not overlap with those of the host basalts.

Zircon U-Pb geochronology of the Zambales and Angat ophiolites, Luzon, Philippines: evidence for an Eocene arc-back arc pair

Two basement terranes, the Zambales ophiolite in the west, and the Angat ophiolite in the east, are exposed on the island of Luzon, separated by a circa 10 km thick and circa 100 km wide sedimentary basin. The structural and age relationships between the two ophiolitic blocks are central to understanding the geologic and tectonic development of the northern Philippines and evaluating models of terrane evolution proposed for this area of the western Pacific. We analyzed zircons from the Zambales and Angat terranes to better constrain their origin. Two zircon fractions from tonalite in the Acoje block of the Zambales ophiolite give concordant U-Pb ages at 44.2 (±0.9) Ma. Two zircon fractions from plagiogranite and one fraction from diorite in the Coto block of the Zambales ophiolite give concordant U-Pb ages of 45.1 (±0.6) Ma. These results provide a Middle Eocene age for the Zambales ophiolite, in agreement with the minimum Late Eocene age of the overlying Aksitero Formation. No age difference is discerned between the arc-like Acoje block and MORB-like Coto block of the Zambales ophiolite. Four zircon fractions from two sample sites in the Angat ophiolite give concordant ages of 48.1 (±0.5) Ma. This age is considerably younger than the Late Cretaceous age based on radiolarian fauna derived from a sheeted dike-pillow lava-sediment sequence south-southeast of the main ophiolite. The small age difference between the Zambales and Angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the Southern Sierra Madre beneath the Central Valley of Luzon. The Cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the Eocene Angat ophiolite implies that there are two ophiolitic basements exposed in the Southern Sierra Madre. The relationship between the two ophiolites is constrained by the overlying stratigraphic relations which indicate that an Eocene volcanic arc and associated volcaniclastic apron was built on both the Eocene and Cretaceous ophiolitic basement. This suggests that the Zambales-Angat ophiolite represents a preserved Eocene back-arc basin that opened behind an Eocene arc that developed within Cretaceous oceanic basement. In this model, the Zambales-Angat ophiolites are therefore not allochthonous terranes but part of a single plate, generated in situ, forming part of the autochthonous basement of Luzon.