Rocks Of Similar Age Research Articles

The Border Ranges fault system in Glacier Bay National Park, Alaska: evidence for major early Cenozoic dextral strike-slip motion

The Border Ranges fault system of southern Alaska, the fundamental break between the arc basement and the forearc accretionary complex, is the boundary between the Peninsular–Alexander–Wrangellia terrane and the Chugach terrane. The fault system separates crystalline rocks of the Alexander terrane from metamorphic rocks of the Chugach terrane in Glacier Bay National Park. Mylonitic rocks in the zone record abundant evidence for dextral strike-slip motion along north-northwest-striking subvertical surfaces. Geochronologic data together with regional correlations of Chugach terrane rocks involved in the deformation constrain this movement between latest Cretaceous and Early Eocene (~50 Ma). These findings are in agreement with studies to the northwest and southeast along the Border Ranges fault system which show dextral strike-slip motion occurring between 58 and 50 Ma. Correlations between Glacier Bay plutons and rocks of similar ages elsewhere along the Border Ranges fault system suggest that as much as 700 km of dextral motion may have been accommodated by this structure. These observations are consistent with oblique convergence of the Kula plate during early Cenozoic and forearc slivering above an ancient subduction zone following late Mesozoic accretion of the Peninsular–Alexander–Wrangellia terrane to North America.

Rapid magma production rates, underplating and remelting in the Andes: isotopic evidence from northern-central Peru (9–11 °S)

Combined strontium and neodymium isotope data on the Miocene-Pliocene Cordillera Blanca batholith (9–11 °S) are compared with acid plutonic and volcanic rocks of similar age and composition from the central Andes (ca. 16–23 °S). Although intruded through 50–60 km of continental crust the batholith rocks, which range in composition from quartz diorite to high silica leucogranodiorite, show little sign of contamination by mature continental basement. Initial 87Sr 86Sr ratios define a range of 0.7041 to 0.7057, with average ϵ Nd values close to bulk earth (−0.5), over a relatively large range in SiO 2. It is difficult to reconcile the isotopic composition of the batholith rocks with simple AFC models involving fractionation of either clinopyroxene (deep-level) or plagioclase (high-level) dominated assemblages, and the isotopic variation in these rocks is instead considered to be inherited from a primary subcontinental lithosphere source through a two-stage process of crustal underplating and subsequent partial melting. Estimated (mantle) magma production rates during underplating are 0.1–0.3 km 3 yr −1. Comparisons with Miocene plutonic rocks from the western and eastern Cordillera at ~ 11 °S show no clear trends in isotopic compositions in time or space. Furthermore, the isotopic compositions of Nd and Sr in the batholith rocks contrast strongly with basaltic to high silica volcanic and plutonic rocks of similar age exposed in the central Andes, where 143Nd 144Nd and 87Sr 86Sr are thought to reflect contamination of mantle-derived magmas and/or tectonic reworking of old basement material during crustal thickening. The lack of significant involvement of basement material in the petrogenesis of the batholith magmas may be due to differences in the mechanism of crustal thickening along the Andean chain during the Late Miocene, with thickening in the central Andes being caused predominantly by tectonic shortening, while in northern-central Peru thickening occurred mostly through crustal underplating of mantle-derived basalt. These two regions thus represent first-order crustal domains in the central Andean Cordilleras. The boundary between them is marked by the Abancay Deflection.

Ion microprobe dating of Paleozoic granitoids: Devonian magmatism in New Zealand and correlations with Australia and Antarctica

Precise ion microprobe UPb zircon ages have been obtained from a representative set of Paleozoic igneous rocks from the Western Province of the South Island, New Zealand. Granitoid rocks forming the Karamea Batholith and related plutons in the Buller terrane all yield crystallisation ages that are indistinguishable within a ± 5 Ma uncertainty at 375 Ma (Middle-Late Devonian). Previous workers have suggested that the batholith was emplaced over a long time interval and comprises rocks ranging in age from 370 to 310 Ma, with the bulk in the Early Carboniferous. Granitoid rocks with Carboniferous ages (∼ 330 Ma) do occur further to the west and younger Cretaceous granitoids occur within the Karamea Batholith, but these are not volumetrically significant. A sample from the ultramafic-mafic Riwaka Complex in the adjacent Takaka terrane gave a crystallisation age of 376.9 ± 5.6 Ma (2σ), indicating emplacement coeval with the Karamea Batholith. The Paleozoic granitoids contain a large amount of inherited zircon, with distinct 390-, 500–600- and 1000-Ma components. The 390-Ma age corresponds to widespread plutonism in the Lachlan Fold Belt in SE Australia. The 500–600-Ma (Ross-Delamerian age) and 1000-Ma (Grenville age) age components have also been observed in granitoids from the Lachlan Fold Belt and in Ordovician metasedimentary rocks from SE Australia and New Zealand. The inherited zircons in the Karamea Batholith could be derived from continental basement at depth, from the incorporation of upper-crustal material in the granitoid magmas or both. The Devonian granitoids in New Zealand can now be correlated with rocks of similar age in northern Victoria Land (Admiralty Intrusives) and Marie Byrd Land (Ford Granodiorite) in West Antarctica, in NE Tasmania, and in the central part of the Lachlan Fold Belt in SE Australia. On a reconstruction of the SW Pacific margin of Gondwana, prior to later break-up, it is possible to trace out a semi-continuous magmatic belt in excess of 2000 km in length.

Magmatic History of the Avalon Terrane of Southern New Brunswick, Canada, Based on U-Pb (Zircon) Geochronology

U-Pb (zircon) dates from volcanic and plutonic units confirm that igneous activity throughout the Caledonian Highlands of southern New Brunswick occurred in two main episodes: ca. 620 Ma and ca. 560-550 Ma. The ca. 620 Ma episode is represented by mainly metatuffaceous rocks of the Broad River Group and associated dioritic to granitic plutons that form most of the eastern highlands. Igneous rocks of similar age and type have been documented in Avalonian areas throughout the northern Appalachian Orogen and are interpreted generally to have formed during subduction at a continental margin. In contrast, the ca. 560-550 Ma igneous episode may be unique to the Caledonian Highlands. It is represented by tuffs and bimodal flows of the Coldbrook Group and co-genetic gabbroic and granitic plutons that occur mainly in the western highlands. This voluminous magmatism may have taken place in a volcanic arc and intra-arc extensional setting, perhaps like that represented by older but lithologically similar rocks of th...

Ion microprobe UPb zircon geochronology of granitic magmatism in the Western Province of the South Island, New Zealand

Zircon UPb isotopic systems are described for three mid-Palaeozoic and four late Mesozoic granites from the Western Province of the South Island, New Zealand. The granites chosen are representative of the major plutons in this region. Two distinct episodes of Palaeozoic magmatism have been identified: Middle Devonian (∼ 380 Ma) and Early Carboniferous (∼ 330 Ma). The Mesozoic granites are all mid-Cretaceous in age, and range from ∼ 120 to ∼ 110 Ma. The O'Sullivans Granite, forming a large part of the Karamea Batholith, has a crystallisation age of 377.7 ± 4.1 Ma, and the Windy Point Granite from the Paparoa Batholith has an age of 328.6 ± 4.4 Ma. Further west, the granite at Cape Foulwind has a similar age of 327.3 ± 6.2 Ma. From east to west, the Cretaceous granites appear to decrease in age. The Separation Point Batholith is dated at ∼ 118 Ma. The oldest phase at Separation Point gave an age of 116.6 ± 1.9 Ma and the youngest phase, the Pearse Granodiorite, gave the same age within error at 119.4 ± 2.3 Ma. The Mount Olympus pluton in NW Nelson gave an age of 111.4 ± 2.0 Ma and the same age is found for the Buckland Granite, which forms most of the Paparoa Batholith, at 109.6 ± 1.7 Ma. Inherited zircon components as old as 1700 Ma have been identified in many of the granites, and distinct components are present at 1000 and 500–600 Ma. This pattern of zircon ages has also been observed in granites from the Lachlan Fold Belt and in Ordovician sediments from SE Australia and New Zealand. However, it is not clear whether the old zircons are derived from the lower crust, from the incorporation of upper-crustal material, or both. The Palaeozoic granites can be broadly correlated with rocks of similar age in Antarctica, SE Australia and Tasmania that formed part of the Gondwana supercontinent. The Cretaceous granites are probably part of an extensive Mesozoic magmatic arc that can be traced from South America through West Antarctica and on into New Zealand.

An enigmatic cap-shaped fossil from the Middle Cambrian of North Greenland

Nyeboeconus robisoni gen. et sp. nov., is described from the Middle Cambrian Henson Gletscher Formation of western North Greenland. Some authors have interpreted similar shelIs as chondrophorine hydrozoans or invertebrate fossils of uncertain systematic position. The coiled, cap-shaped shell and the presence of an internal plate, or pegma, suggest, however, that this new form is the second genus to be described of the Family Enigmaconidae MacKinnon, 1985 (Mollusca, Class Helcionelloida), otherwise known only from rocks of similar age in New Zealand.

Tectonic implications of paleomagnetism in Upper Cretaceous deposits in the Haţeg and Rusca Montanā basins (South Carpathians, Romania)

Previous paleomagnetic studies of Upper Cretaceous deposits in the Haţeg Basin (26 sites) and Rusca Montanā Basin (four sites) concluded that only a few of these sites contained useful paleomagnetic information. For this reason, we utilized only data from six sites in Haţeg Basin (45.5°N, 22.8°E) and two sites in Rusca Montanā Basin (45.5°N, 22.4°E) for further analysis. After AF demagnetization, we identified the primary characteristic remnant magnetizations, which yielded a mean direction of D = 76.9°, I=32.9° and a mean pole of φ = 21.8°N, λ = 109.0°E, A 95= 14.2°. A comparison with the paleomagnetic results obtained from rocks of similar age from stable Africa and Europe shows that to reach their present-day position the studied areas moved to the north and rotated to the east. These results are consistent with the other paleomagnetic data in the Upper Cretaceous magmatic belt of Romania.

Tectonics and heat sources for granulite metamorphism of supracrustal-bearing terranes

There are generally three important processes in the granulite-facies metamorphism of supracrustal rocks: (1) transport from the surface to 15–30+ km, (2) heating to 700–800+ °C, and (3) transport back to the surface and re-exposure at the top of a crust of “normal” thickness (35–40 km). Mantle-derived magmatism, and/or magmatic and/or metasomatic fluid flow, can transport the necessary heat for granulite metamorphism, but many granulite terranes do not contain sufficient volumes of mafic intrusive rocks, or evidence of extensive fluid flow, to readily explain heating by this mechanism. One possible process for the formation of large granulite terranes involves continental collision and crustal doubling by simple-shear thrusting. Heating of the underthrust supracrustal rocks would occur by thermal relaxation, but, if granulite-facies temperatures are to be attained at mid-crustal levels, this mechanism necessarily produces extensive melting of the lower crustal plate, suggesting that granulite terranes formed in this way should contain substantial syn- or post-metamorphic crustally-derived intrusions. In this situation, subsequent uplift and re-exposure at the surface is a natural consequence of erosion and isostatic readjustment. An alternative process which may allow the heating of supracrustal rocks to granulite-facies temperatures is imbricate thrusting in which thin slivers or “flakes” of supracrustal rocks are thrust down to the lowermost crust. This mechanism requires a separate tectonic event, such as a second thrusting event, to transport the granulite-facies rocks back to the surface. Other possible heat sources for supracrustal granulite terranes include preheating of the overthrust crust by arc or pre-orogenic magmatism, and shear heating on thrust planes. Models indicate that pre-heating of the overthrust plate has only minor thermal effects upon the thermal history of the underthrust crust, although shear heating may be significant. Heat sources for granulite metamorphism of the Archean Limpopo Belt of South Africa are considered in terms of possible tectonic mechanisms. The metamorphic grade and presence of abundant supracrustal rocks in this terrane demonstrate that it was once at the Earth's surface, buried to ∼ 8.5 kbar (∼ 28 km) and heated to ∼ 850°C, then re-exposed at the surface. There are no recognizable mantle-derived igneous rocks of suitable age, and no evidence for extensive fluid flow that could have provided a magmatic heat source for the metamorphism. Shear heating is unlikely because the pattern of metamorphic isograds is not related to faults or shear zones. Imbricate thrusting of thin silvers of supracrustal rocks into the lowermost crust to allow heating without magmatism would require a separate tectonic event to transport the granulites to the surface; there is no evidence for such a later event. The most satisfactory and self-consistent model for Limpopo metamorphism involves thermal relaxation of a perturbed temperature profile produced by Tibetan-style continental collision of the Zimbabwe and Kaapvaal Cratons, with consequent crustal doubling to at least 65 km thick, and the generation of crustal melts which advected heat into the zone of granulite metamorphism. There is no evidence for significant pre-collisional heating of either crustal plate; both probably had typical “shield-type” pre-collisional geothermal gradients. Crustally-derived magmas, therefore, must have been produced during thermal and structural equilibration of the Limpopo collision. Abundant granitoid rocks of similar age to the metamorphism occur both as migmatites and as discrete plutons.

Rhinns complex: A missing link in the Proterozoic basement of the North Atlantic region

Paleoproterozoic basement occurs on the islands of Islay and Colonsay in western Scotland and on Inishtrahull off the north coast of Ireland. The basement is composed of a deformed alkalic igneous association, collectively referred to as the Rhinns complex. Isotopic data indicate that the complex represents new addition of mantle-derived material to the crust at ca. 1.8 Ga. The Rhinns complex may link with rocks of similar age in North America, Greenland, and Scandinavia, which formed part of the southern margin of the Laurentia-Baltica supercontinent.

Osmium-isotope ratios of platinum-group minerals associated with ultramafic intrusions: Os-isotopic evolution of the oceanic mantle

Osmium-isotope ratios were determined by an ion microprobe on the individual platinum-group minerals (PGM) from placers, which are associated with ultramafic intrusions of late Precambrian to Tertiary age. Unlike Os-isotope ratios in large layered mafic intrusions, these 187Os/ 186Os ratios are low, and within a narrow range from 0.99 to 1.12, which is attributed to the occurrences of the intrusions. There was no opportunity to incorporate old crustal Os because of the small sizes of the intrusions and the mode of emplacement into the upper crustal level. In addition, the interaction with the host volcanic rocks of similar age, if any, would not have seriously affected the 187Os/ 86Os ratios of the peridotites. While different phases of PGM in one grain have similar 187Os/ 186Os ratios, there is a significant variation in a given district. The variation is attributed to a long-term heterogeneity in Re/Os ratios of the oceanic upper mantle. The lowest value in each area is lower than the value expected from the evolution of bulk Earth composition. The lowering may be due to primordially low Re/Os ratios in the mantle or preferential removal of Re by partial melting to form the continental crust. The former model is rejected because most chondrites have higher Re/Os ratios than type C1 and the core-mantle separation would not have lowered Re/Os ratios. The low 187Os/ 186Os ratios are, therefore attributed to the extraction of continental crust by preferential removal of Re from the mantle through partial melting. The model is consistent with the depleted nature of oceanic peridotites (positive ε Nd , negative ε Sr , and low Re/Os ratios). Calculations of 187Os/ 186Os ratios of the mantle residue suggest that the observed data are in accordance with a model involving the extraction of ∼ 2% melt by fractional fusion from the mantle of C1 chondritic composition at ∼ 2.0 Ga. If the bulk Earth has higher Re/Os ratios, as proposed by Martin [1], then the observed data require much larger degrees of partial melting or older mean age of partial melting at ∼ 3.0 Ga. The variation in Re/Os ratios in each area is ascribed to the various degrees of mixing of the depleted mantle source and more fertile material. This may be due to local heterogeneity in the mantle, “marble-cake” like mantle [2] or replenishment of Re and Os from a more fertile source by sulphide ( + CO 2) in the mantle. Generally higher 187Os/ 186Os ratios found in this study compared with those from the southern African sub-continental lithosphere [3] may be attributed to the nature of the latter which is more isolated than the oceanic mantle from the rest of the mantle. The occurrence of S in the mantle may influence Re Os isotope variations and the behavior of S may decouple the Re Os system from other lithophile radiogenic isotope systems.

Rapid production and evolution of late Archaean felsic crust in the Vestfold Block of East Antarctica

The aim of this study is to present a definitive chronostratigraphy, based for the first time on Ub zircon ages, for the Vestfold Block of East Antarctica. This region consists of four principal rock units, three of which are composed of felsic orthogneisses, and form the subject of this article. These orthogneiss units comprise the previously defined Crooked Lake and Mossel gneisses, and a newly proposed felsic unit, the Grace Lake granodiorite. The entire gamut of significant felsic igneous activity, the two intense tectonothermal events which preceded the cratonisation of the Vestfold Block, and significant post-tectonic uplift were all accomplished during an approximately 50 Ma interval at the end of the Archaean. This conclusion is in marked contrast to a previously proposed model that such activity had occurred episodically over several hundred million years. The Mossel gneiss (predominantly tonalitic orthogneiss) has the oldest igneous precursors, with a significant age spread, from 2526±6 Ma (in the type area) to 2501±4 Ma. Precursors of the compositionally varied Crooked Lake gneiss (predominantly dioritic, monzodioritic and monzonitic rocks) also show a distinct range of crystallisation ages, from 2501±4 Ma in the type area, through 2493±5 Ma to 2484±6 Ma. Both units appear to show a general northwards migration of igneous activity. The Grade Lake granodiorite, which is defined here on field and isotopic criteria, yields a pooled mean age (from two rocks) of 2487±6 Ma. An undeformed, post-granulite-facies quartz diorite dyke was emplaced at 2477±5 Ma. The new isotopic data constrain the age of the first major granulite-facies deformation (D 1) to have affected the Vestfold Block to lie between 2501±4 Ma and 2487±6 Ma, and that of the second granulite-facies deformation (D 2) to 2487±6 Ma. There is evidence of older precursors for some of the late Archaean gneisses. Zircon cores and xenocrysts together with SmNd model ages indicate that the Grace Lake granodiorite was derived from crustal sources at least 2800 Ma old. The presence of inherited zircon in one of two analysed samples of the Mossel gneiss, and a spread of SmNd model ages, implies derivation by the melting of at least two source components, one at least 2700 Ma old. In contrast, the apparent lack of inherited zircon in the Crooked Lake gneiss samples is consistent with derivation of their precursors either exclusively from the mantle, or by a brief two-stage melting process. The Vestfold Block chronology is completely different from that of the adjacent Rauer Islands region, indicating that they were not juxtaposed during Archaean times. Neither do any nearby Archaean terranes appear to have comparable chronostratigraphy with the Vestfold Block. At our present state of knowledge, only the Napier Complex of Enderby Land, and possibly the southern Prince Charles Mountains of MacRobertson Land contain any rocks of similar age. This makes the Vestfold Block a potentially useful element for theoretical reconstruction of Gondwanaland.

Provenance of the Bonner Formation (Belt Supergroup), Montana: Insights from U-Pb and Sm-Nd analyses of detrital minerals

Individual detrital monazite grains from the Bonner Formation of the Belt Supergroup in southwestern Montana have been analyzed using U-Pb and Sm-Nd isotopes. Combined with single-grain U-Pb analyses of detrital zircon, these data place constraints on the age and isotopic characteristics of the source area, previously inferred to be the west and south of the Belt basin. U-Pb ages from eight single zircon grains range from 1.70 to 1.86 Ga; two grains were ca. 2.6 Ga. U-Pb ages from five monazite grains range from 1.64 to 1.79 Ga. Sm-Nd analyses of these same monazite grains yield {epsilon}{sub Nd} at the time of crystallization of +0.7 to {minus}14.5. These data suggest that the source area for this part of the Bonner Formation was an Early Proterozoic magmatic (orogenic ) belt with some reworked Archean material. It is postulated that this source area may have been the western edge of Archean crust (Wyoming craton) that was involved in younger magmatism, possibly continuous with rocks of similar age in south-central Montana, the subsurface of Alberta, or the southwestern United States.

Paleomagnetism and geochronology of late Paleozoic granitic rocks from the Lake District of southern Chile:Implications for accretionary tectonics

Research Article| April 01, 1991 Paleomagnetism and geochronology of late Paleozoic granitic rocks from the Lake District of southern Chile:Implications for accretionary tectonics Myrl E. Beck, Jr.; Myrl E. Beck, Jr. 1Department of Geology, Western Washington University, Bellingham, Washington 98225 Search for other works by this author on: GSW Google Scholar Alfredo Garcia R.; Alfredo Garcia R. 1Department of Geology, Western Washington University, Bellingham, Washington 98225 Search for other works by this author on: GSW Google Scholar Russell F. Burmester; Russell F. Burmester 1Department of Geology, Western Washington University, Bellingham, Washington 98225 Search for other works by this author on: GSW Google Scholar Francisco Munizaga H.; Francisco Munizaga H. 2Departamento de Geología y Geofísica, Universidad de Chile, Casilla 13518 Correo 21, Santiago, Chile Search for other works by this author on: GSW Google Scholar Francisco Hervé A.; Francisco Hervé A. 2Departamento de Geología y Geofísica, Universidad de Chile, Casilla 13518 Correo 21, Santiago, Chile Search for other works by this author on: GSW Google Scholar Robert E. Drake Robert E. Drake 3Institute for Human Origins/Berkeley Geochonology Center, 2453 Ridge Road, Berkeley, California 94709 Search for other works by this author on: GSW Google Scholar Geology (1991) 19 (4): 332–335. https://doi.org/10.1130/0091-7613(1991)019<0332:PAGOLP>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Myrl E. Beck, Alfredo Garcia R., Russell F. Burmester, Francisco Munizaga H., Francisco Hervé A., Robert E. Drake; Paleomagnetism and geochronology of late Paleozoic granitic rocks from the Lake District of southern Chile:Implications for accretionary tectonics. Geology 1991;; 19 (4): 332–335. doi: https://doi.org/10.1130/0091-7613(1991)019<0332:PAGOLP>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract K/Ar and 40Ar/39Ar dating shows that granitic rocks in the central Lake District, southern Chile, are late Paleozoic. Granitic rocks of similar age extend far up the coast of Chile, perhaps as much as 1800 km. We have obtained paleomagnetic directions from seven sites near Lago Ranco; these give a paleomagnetic pole at lat 57.4°S, long 323.5°E, with a circle of 95% confidence of 18.8°. This is not significantly different from the mean late Paleozoic reference pole for the South American craton. If our results are characteristic of the late Paleozoic belt as a whole, it follows that post-late Paleozoic accretion and terrane displacement are absent throughout much of the central Andes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonics and volcanism in the Sado-Pohang Belt from 20 to 14 Ma and opening of the Yamato Basin of the Japan Sea

Tertiary volcanism around the Japan Sea is closely related to the opening of the Japan Sea. Paleogene to early Middle Miocene formations in the Pohang-Yangnam area of the Korean Peninsula mainly consist of volcanic rocks that are stratigraphically and petrographically similar to similar age rocks on the Japan Sea side of the Japanese Islands, especially those of Sado Island. We propose that all these early Cenozoic rocks formed part of a single tectonic-volcanic belt called Sado-Pohang Belt. It is inferred that this belt was located on the south side of the Yamato Ridge in Paleogene time. After the Japan Basin opened, the Yamato Basin opened during Early Miocene to the early Middle Miocene time and part of the Sado-Pohang Belt migrated southward. The tectonics of the Sado-Pohang Belt is considered to support the hypothesis of the two-stage opening of the Japan Sea.

Problematic bedding-plane markings from the Middle Proterozoic Manganese Subgroup, Bangemall Basin, Western Australia

Enigmatic bedding-plane markings, impressions resembling isolated strings of beads, occur in sandy and argillaceous strata of the Stag Arrow Formation in the Middle Proterozoic Manganese Subgroup, Bangemall Group, of the Bangemall Basin, east of Newman, Western Australia. Markings are abundant at two localities and traces are found at a third. Structures similar in both morphology and dimension have previously been reported from the Appekunny Formation in the Middle Proterozoic Belt Supergroup of U.S.A. The Manganese Subgroup structures provide further details of morphology, enabling comparison with a variety of biogenic and non-biogenic structures. A source for the markings still cannot be determined with certainty, but a biogenic origin is favoured, and affinities with the green or brown algae suggested. Such an origin is consistent with records of megascopic algae from rocks of similar age.

Paleomagnetism of Middle Miocene volcanic rocks in the Mojave‐Sonora Desert region of western Arizona and southeastern California

Paleomagnetic directions have been obtained from 190 early to middle Miocene (12–20 Ma) mafic volcanic flows in 16 mountain ranges in the Mojave‐Sonora desert region of western Arizona and southeastern California. These flows generally postdate early Miocene tectonic deformation accommodated by low‐angle normal faults but predate high‐angle normal faulting in the region. After detailed demagnetization experiments, 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations (TRM). Because of the episodic nature of basaltic volcanism in this region, the 179 flows yielded only 65 time‐distinct virtual geomagnetic poles (VGPs). The angular dispersion of the 65 VGPs is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation. The paleomagnetic pole calculated from the 65 cooling unit VGPs is located at 85.5°N, 108.9° within a 4.4° circle of 95% confidence. This pole is statistically indistinguishable (at 95% confidence) from reference poles calculated from rocks of similar age in stable North America and from a paleomagnetic pole calculated from rocks of similar age in Baja California. The coincidence of paleomagnetic poles from the Mojave‐Sonora desert region with reference poles from the stable continental interior indicates that (1) significant vertical axis net tectonic rotations have not accompanied post‐middle Miocene high‐angle normal faulting in this region; (2) there has been no detectable post‐middle Miocene latitudinal transport of the region; and (3) long‐term nondipole components of the middle Miocene geomagnetic field probably were no larger than those of the recent (0–5 Ma) geomagnetic field. In contrast, paleomagnetic data indicate vertical axis rotations of similar age rocks in the Transverse Ranges, the Eastern Transverse Ranges, and the Mojave Block. We speculate that a major structural discontinuity in the vicinity of the southeastward projection of the Death Valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations.

Paleomagnetism of the Kvarner islands, Yugoslavia

Nearly 400 samples were drilled at 44 localities from Cretaceous and Eocene carbonates and Eocene flysch, all in para-autochthonous position. As a result of thermal cleaning most of the Cretaceous localities yielded statistically well-defined mean directions. Thus overall means were calculated for the Cretaceous of Cres and Krk islands, respectively (Cres, six localities: D = 308°, I = 45°, k = 26, α 95 = 13.3°, D c = 330°, I c = 48°, k = 33, α 95 = 11.9°; Krk, seven localities: D = 342°, I = 42°, k = 12, α 95 = 17.8°, D c = 348°, I c = 48°, k = 26, α 95 = 12.1°). These directions, corrected for tilt, agree well with those determined for rocks of similar age from the fold belt north of “autochthonous’ Istria, the Trieste Karst and the Southern Alps, while exhibiting clockwise rotation of about 30° with respect to the rigid part of the Adriatic region. The natural remanent magnetization (NRM) of most of the Eocene sediments was either too weak to be cleaned properly or became erratic before characteristic remanent magnetization (ChRM) could be isolated. The mean directions of the Eocene localities with ChRM differ so much, both before and after tectonic correction, that an overall mean was not calculated. Despite the scatter, certain trends could be recognized. Counter-clockwise rotation characterizes most of the Eocene sites. The two sites in Krk with clockwise rotation may belong not to the Dalmatian but the High Karst zone.

Petrological affinities of intrusive rocks associated with the giant mesothermal gold deposit at Porgera, Papua New Guinea

Study of the Miocene Porgera intrusive rocks over 40 yr has generated at least four mutually contradictory classification schemes. These rocks are here reassessed, using a computer database of 256 analyses of volcanic and intrusive rocks of similar age from Papua New Guinea (PNG), and 100 analyses of the Porgera rocks themselves. The Porgera mafic rocks are not ordinary basalts, andesites, gabbros or diorites, because they carry abundant titanian amphibole, diopsidic clinopyroxene, and have peculiar (e.g. panidiomorphic, glomeroporphyritic, globular) textures. As a suite, they are too rich in LILE (Sr, K, Rb, Ba) and HFS (Th, Nb, Ce, P, Zr, Ti) elements, too poor in Y, and include too many silica-undersaturated compositions, to be termed tholeiitic or calc-alkaline. They are also too potassic, too poor in HFS (especially Ti, Y), too rich in Al, and include too many oversaturated compositions, to be termed alkaline. Combining their position on standard classification diagrams (e.g. TAS, K 2O-SiO 2) with their mineralogy and texture suggests that the suite is shoshonitic and, more specifically, appinitic/lamprophyric. Thus they show marked affinites with contemporaneous shoshonitic rocks in PNG, but differ most notably in their high Nb, Ce and Th, their sodi-potassic character, and their peculiar texture. Porgera thus becomes one of an increasing array of space/time associations throughout the geological record and between lamprophyric magmatism and major gold deposits.

Pteropods (Mollusca: Gastropoda) from Tertiary formations of Washington and Oregon

Praehyalocylis cretacea (Blanckenhorn, 1889), a pteropod previously known only from upper Eocene to middle Miocene strata in Europe and Turkey, is reported for the first time in similar age rocks in the northwestern United States. Of the 238 specimens, most occur as molds and casts in concretions in deep-water deposits from the Keasey Formation in Oregon, and from the Quimper Sandstone, Blakeley Formation, Pysht Formation, and Astoria Formation of Washington. Praehyalocylis has not been reported previously from the Western Hemisphere.Clio berglundi n. sp. and C. goederti n. sp. are reported from upper Oligocene to lower Miocene rocks in Washington. Eleven specimens were found mostly as internal molds in concretions in deep-water deposits of the Lincoln Creek, Pysht, and Astoria Formations of Washington. Cenozoic species of Clio have not been reported previously from the West Coast of the United States.

Palaeomagnetism of the Upper Carboniferous Strzegom and Karkonosze Granites and the Kudowa Granitoid from the Sudet Mountains, Poland

Carboniferous granites and granitoids, varying in age from 305 to 281 Ma, from the Sudet mountains (latitude 51N, longitude 16E), Poland, carry dual-polarity magnetic components consisting of an inferred primary high-temperature A-component found in the Strzegom Granite, the Karkonosze Granite with the bordering Karpacz Hornfels and the Kudowa Granitoid and scattered low-stability components of Mesozoic-Tertiary age present in all rocks except the Strzegom Granite. The group-A component defines a palaeomagnetic pole position ( D = 202, I = − 12, α 95 = 13.0, N = 4, calculated palaeomagnetic pole position 42°S, 346°E) which is in good agreement with palaeomagnetic results from rocks of similar age in western Europe, thus implying that no major tectonic movement has taken place in the Sudets relative to stable Europe since Upper Carboniferous times.