Wayne R. Premo

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Precambrian ophiolites of arabia: geologic settings, UPb geochronology, Pb-isotope characteristics, and implications for continental accretion

Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740–780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820–870 Ma and

Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions

1250 Ma. The 820–870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the nothern end of the 900 km-long Nabitah mobile belt was dated by the zircon UPb method at ∼ 830 Ma. This date is in the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture, but is ∼ 100 Ma older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq suture that was transposed parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data from the ophiolites are of three types: (1) lead from the ophiolitic rocks and arc tonalites of the northwestern Shield and ophiolitic rocks of the Nabitah suture is similar to lead in modern mid-ocean ridge basalt, (2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and (3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous and perhaps primitive oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.

Evolution of the Early Proterozoic Colorado province: Constraints from U-Pb geochronology

Precise U-Pb baddeleyite dating of mafic igneous rocks provides evidence for a widespread and synchronous magmatic event that extended for >2400 km along the western margin of the Neoproterozoic Laurentian craton. U-Pb baddeleyite analyses for eight intrusions from seven localities ranging from the northern Canadian Shield to northwestern Wyoming–southwestern Montana are statistically indistinguishable and yield a composite U-Pb concordia age for this event of 780.3 ± 1.4 Ma (95% confidence level). This 780 Ma event is herein termed the Gunbarrel magmatic event. The mafic magmatism of the Gunbarrel event represents the largest mafic dike swarm yet identified along the Neoproterozoic margin of Laurentia. The origin of the mafic magmatism is not clear, but may be related to mantle-plume activity or upwelling asthenosphere leading to crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the proto– Pacific Ocean. The mafic magmatism of the Gunbarrel magmatic event at 780 Ma predates the voluminous magmatism of the 723 Ma Franklin igneous event of the northwestern Canadian Shield by ∼60 m.y. The precise dating of the extensive Neoproterozoic Gunbarrel and Franklin magmatic events provides unique time markers that can ultimately be used for robust testing of Neoproterozoic continental reconstructions.

A precise 232Th-208Pb chronology of fine-grained monazite: Age of the Bayan Obo REE-Fe-Nb ore deposit, China

The Colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the Archean Wyoming craton during the Early Proterozoic, between about 1790 and 1660 Ma. Correspondence in ages between metamorphism, deformation, and plutonism; association of volcanic rocks with comagmatic calc-alkalic plutons; and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton.

Age and origin of the Tanghu Granite, southeast China: Results from U-Pb single zircon and Nd isotopes

Abstract We have obtained precise Th-Pb internal isochron ages on monazite and bastnaesite for the worlds largest known rare earth elements (REE)-Fe-Nb ore deposit, the Bayan Obo of Inner Mongolia, China. The monazite samples, collected from the carbonate-hosted ore zone, contain extremely small amounts of uranium (less than 10 ppm) but up to 0.7% ThO2. Previous estimates of the age of mineralization ranged from 1.8 to 0.255 Ga. Magnetic fractions of monazite and bastnaesite samples ( 232 Th 204 Pb values (900–400,000) and provided precise Th-Pb internal isochron ages for paragenetic monazite mineralization ranging from 555 to 398 Ma within a few percent error (0.8% for two samples). These results are the first indication that REE mineralization within the giant Bayan Obo ore deposit occurred over a long period of time. The initial lead isotopic compositions (low 206 Pb 204 Pb and high 208 Pb 204 Pb) and large negative ϵNd values for Bayan Obo ore minerals indicate that the main source(s) for the ores was the lower crust which was depleted in uranium, but enriched in thorium and light rare earth elements for a long period of time. Zircon from a quartz monzonite, located 50 km south of the ore complex and thought to be related to Caledonian subduction, gave an age of 451 Ma, within the range of monazite ages. Textural relations together with the mineral ages favor an epigenetic rather than a syngenetic origin for the orebodies. REE mineralization started around 555 Ma (disseminated monazite in the West, the Main, and south of the East Orebody), but the main mineralization (banded ores) was related to the Caledonian subduction event ca. 474-400 Ma.

Isotopic, geochemical, and temporal characterization of Proterozoic basement rocks in the Quitovac region, northwestern Sonora, Mexico: Implications for the reconstruction of the southwestern margin of Laurentia

U-Pb single-zircon and Sm-Nd whole-rock isotopic data for the Tanghu Granite of the Zhuguangshan batholith suggest that a basement either composed of or derived from Middle Proterozoic to Late Archean rocks underlies southeast China The granite contains two kinds of inherited zircon components, whole anhedral zircons and small cores within younger magmatic zircons. The U-Pb data for a single analysis of four magmatic zircons yielded a concordant age of 433 ±2 Ma, whereas single zircons yielded two linear arrays with lower intercepts of 435 ±1 and 434 ±14 Ma and upper intercepts of 1415 ±3 and 2516 ±6 Ma, respectively. Xenocrystic zircons of Archean age had not previously been recognized in any rocks from southeast China Two whole-rock samples from the granite yielded ϵNd values of -7.1 and -8.3 at 434 Ma and Nd model ages of ∼1750 Ma relative to a depleted mantle. We conclude from this isotope data that the Tanghu Granite crystallized at 434 ±1.6 Ma (weighted average) and was derived from a source of unknown age, but had an average crustal-residence age of ∼1750 Ma, and was partially composed of a mixture of continental crustal material of at least two ages (∼1400 and ∼2500 Ma).

Arabian Shield ophiolites and Late Proterozoic microplate accretion

A detailed geochemical characterization of 19 representative Proterozoic basement rocks in the Quitovac region in northwestern Sonora, Mexico, has identified two distinct Paleoproterozoic basement blocks that coincide spatially with the previously proposed Caborca and “North America” blocks. New U-Pb zircon geochronology revises their age ranges, the Caborca (1.78–1.69 Ga) and “North America” (1.71–1.66 Ga) blocks at Quitovac, and precludes a simple age differentiation between them. In addition, Grenvillian-age granitoids (ca. 1.1 Ga), spatially associated with the Caborca block have been identified at Quitovac. Nd isotopes and major- and trace- element geochemistry support the distinction of these Paleoproterozoic blocks. Granitoids of the “North America” block are characterized by depleted eNd values (3.4–3.9) and younger Nd model ages (1800–1740 Ma) and have lower K 2 O, Y, Rb, Ba, Th, REE, and Fe/Mg values than coeval rocks of the Caborca block. The Caborca block granitoids are likewise characterized by slightly less depleted eNd (0.6–2.6) and older Nd model ages (2070–1880 Ma). Despite the subtle differences, granitoids from both the Caborca and “North America” blocks exhibit island arc-like affinities. We propose that the Proterozoic basement rocks from the Quitovac region are an extension of the Proterozoic crustal provinces in the southwestern United States. Specifically, rocks of the Caborca block exhibit an affinity to rocks of either the Yavapai province or the Mojave– Yavapai transition zone, whereas rocks of the “North America” block have signatures similar to those of the Mazatzal province or possibly the Yavapai province of Arizona. The new isotopic ages and geochemical data do not support the existence of the Late Jurassic Mojave–Sonora megashear at Quitovac, as originally proposed. However, the Quitovac region accounts only for a small fraction of the Proterozoic basement in Sonora, so these findings do not eliminate the possibility of a megashear elsewhere in northern Sonora. Our new data create the possibility of alternative hypotheses for the distribution of Paleoproterozoic crustal provinces in southwestern North America that affect reconstructions of the original southwestern margin of Laurentia, and reduce uncertainties in the configuration, timing, and existence of the Proterozoic supercontinent, Rodinia.

Contrasting Proterozoic basement complexes near the truncated margin of Laurentia, northwestern Sonora–Arizona international border region

Fragments of Late Proterozoic ocean crust and mantle (ophiolites) occur within six major fault zones that mark sutures between crustal blocks (microplates) that were accreted between about 630 and 715 Ma to form the Arabian Shield. We report new U-Pb zircon ages for ophiolitic gabbro, diorite, and plagiogranite that range from 840 to 700 Ma and establish these complexes among the oldest proven ophiolites. By dating the ophiolitic rocks we are able to monitor the magmatic ages of sea-floor spreading events during accretion of the Arabian Shield. Comparison of the ophiolitic dates with the ages of the adjacent crustal blocks provides a more complete basis for plate-tectonics reconstruction of the shield than has previously been possible. Our new zircon ages confirm earlier Sm-Nd mineral isochrons for ophiolites of the northwestern Arabian Shield, show that the ophiolites are among the oldest oceanic rocks in each terrane, support near- or within-arc tectonic settings for the ophiolites, suggest that the Bir Umq suture extends north along the Nabitah mobile belt into the northern shield, and suggest that older (>1250 Ma) continental crustal material is locally present in the dominantly “oceanic” western shield. We suggest that Indonesia and Alaska provide Phanerozoic analogues for stages in the accretion history.

SHRIMP U-Pb zircon ages for Big Creek gneiss, Wyoming and Boulder Creek batholith, Colorado: Implications for timing of Paleoproterozoic accretion of the northern Colorado province

We utilize new geological mapping, conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses, and whole-rock radiogenic isotope characteristics to distinguish two contrasting Proterozoic basement complexes in the international border region southeast of Yuma, Arizona. Strategically located near the truncated southwest margin of Laurentia, these Proterozoic exposures are separated by a northwest-striking Late Cretaceous batholith. Although both complexes contain strongly deformed Paleoproterozoic granitoids (augen gneisses) intruded into fi ne-grained host rocks, our work demonstrates marked differences in age, host rock composition, and structure between the two areas. The Western Complex reveals a >5-km-thick tilted section of fi nely banded felsic, intermediate, and mafi c orthogneiss interspersed with tabular intrusive bodies of medium-grained leucocratic biotite granite (1696 ± 11 Ma; deepest level), mediumgrained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphyritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and transposition of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies Nourse, J.A., Premo, W.R., Iriondo, A., and Stahl, E.R., 2005, Contrasting Proterozoic basement complexes near the truncated margin of Laurentia, northwestern Sonora–Arizona international border region, in Anderson, T.H., Nourse, J.A., McKee, J.W., and Steiner, M.B., eds., The Mojave-Sonora megashear hypothesis: Development, assessment, and alternatives: Geological Society of America Special Paper 393, p. 123–182, doi: 10.1130/2005.2393(04). For permission to copy, contact editing@geosociety.org. ©2005 Geological Society of America. *janourse@csupomona.edu. 124 J.A. Nourse et al. spe393-04 page 124 metamorphism, when crystalloblastic textures developed. Deformation and regional metamorphism occurred before emplacement of 1.1 Ga(?) mafi c dikes. Throughout the Eastern Complex, meta-arkose, quartzite, biotite schist, and possible felsic metavolcanic rocks comprise the country rocks of strongly foliated mediumand coarse-grained biotite granite augen gneisses that yield mean Pb/Pb ages of 1646 ± 10 Ma, 1642 ± 19 Ma, and 1639 ± 15 Ma. Detrital zircons from four samples of host sandstone are isotopically disturbed; nevertheless, the data indicate a restricted provenance (ca. 1665 Ma to 1650 Ma), with two older grains (1697 and 1681 Ma). The pervasively recrystallized Paleoproterozoic map units strike parallel to foliation and are repeated in south-trending folds that are locally refolded about easterly hinges. Southeasterly lineation developed in augen gneiss and host strata becomes penetrative in local domains of L-tectonite. Regional metamorphism associated with this tectonism persisted until ca. 1590 Ma, as recorded by metamorphic growths within some zircon grains. Mesoproterozoic intrusions that crosscut the Paleoproterozoic metasediments and augen gneisses include coarsely porphyritic biotite granite (1432 ± 6 Ma) and diabase dikes (1.1 Ga?). Emplacement of the granite was accompanied by secondary high-U overgrowths, dated at 1433 ± 8 Ma, on some of the Paleoproterozoic detrital zircons, and apparently was also responsible for resetting the whole-rock Pb isotopic systematics (1441 ± 39 Ma) within these Eastern Complex augen gneisses. Younger plutons emplaced into both Proterozoic basement complexes include medium-grained quartz diorite (73.4 ± 3.3 Ma and 72.8 ± 1.7 Ma), Late Cretaceous hornblende-biotite granodiorite, and Paleogene leucocratic biotite granite. Neogene sedimentary and volcanic strata overlie basement along unconformities that are tilted to the northeast, southeast, or southwest. A brittle normal fault, dipping gently northeast, juxtaposes Tertiary andesite with Paleoproterozoic metasandstone. These relationships suggest that the area shares a common history of mid-Tertiary extension with southwestern Arizona. Later infl uence of the southern San Andreas fault system is implied by multiple dextral offsets of pre-Tertiary units across northwest-trending valleys. Our structural, geochronologic, and isotopic data provide new information to constrain pre–750 Ma Rodinia reconstructions involving southwestern Laurentia. Whole-rock U-Th-Pb and Rb-Sr isotopic systematics in both Paleoproterozoic gneiss complexes are disturbed, however, well-behaved Sm-Nd analyses preserve depleted initial ε Nd values (+2 to +4) that are distinct from the Mojave crustal province, but overlapping with the Yavapai and Mazatzal Provinces of Arizona. The Eastern Complex has the appropriate age and Nd isotopic signature to be part of the Mazatzal Province, but records major tectonism and metamorphism at ca. 1.6 Ga that postdates the Mazatzal orogeny. Deformed granitoids of the Western Complex have “Yavapai-type” ages and ε Nd but display structures discordant to the southwesterly Yavapai trend in central Arizona. The Western Complex lies along-strike with similar-age rocks (1.77 Ga to 1.69 Ga) of the “Caborca block” that have only been studied in detail near Quitovac and south of Caborca. Collectively, these rocks form a northwest-trending strip of basement situated at the truncated edge of Laurentia. The present-day basement geography may refl ect an original oroclinal bend in the Yavapai orogenic belt. Alternatively, the western Proterozoic belt of Sonora may represent displaced fragments of basement juxtaposed against the Yavapai-Mazatzal Provinces along a younger sinistral transform fault (e.g., the Late Jurassic MojaveSonora megashear or the Permian Coahuila transform). Crustal blocks with these specifi c petrologic, geochronologic, and isotopic characteristics can be found in southcentral and northeastern portions of the Australian Proterozoic basement, further supporting a connection between the two continents prior to breakup of the Rodinian supercontinent.