Hypabyssal Intrusions Research Articles

Geology and Petrology of the Nordeste Volcanic Complex, Sao Miguel, Azores

Research Article| December 01, 1980 Geology and Petrology of the Nordeste Volcanic Complex, Sao Miguel, Azores Louis A. Fernandez Louis A. Fernandez 1Department of Earth Sciences, University of New Orleans‘, New Orleans, Louisiana 70122 Search for other works by this author on: GSW Google Scholar Author and Article Information Louis A. Fernandez 1Department of Earth Sciences, University of New Orleans‘, New Orleans, Louisiana 70122 Publisher: Geological Society of America Received: 03 Dec 1979 Revision Received: 16 May 1980 Accepted: 29 May 1980 First Online: 02 Mar 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 © 1980 The Geological Society of America, Inc. GSA Bulletin (1980) 91 (12_Part_II): 2457–2557. https://doi.org/10.1130/GSAB-P2-91-2457 Article history Received: 03 Dec 1979 Revision Received: 16 May 1980 Accepted: 29 May 1980 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Louis A. Fernandez; Geology and Petrology of the Nordeste Volcanic Complex, Sao Miguel, Azores. GSA Bulletin 1980;; 91 (12_Part_II): 2457–2557. doi: https://doi.org/10.1130/GSAB-P2-91-2457 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 SocietyGSA Bulletin Search Advanced Search Abstract The Nordeste volcanic complex of Sao Miguel island is one of the oldest known complexes (4.01 to 0.95 m.y.) of alkaline volcanism in the eastern Azores. It contains about 1,300 m of exposed volcanics and hypabyssal intrusives that have been subdivided into five stratigraphic sequences: lower basalts, Nordeste ankaramites, upper basalts, tristanites, and trachytes. The bulk of the complex is predominantly of basaltic composition and is confined to the three lowermost sequences: lower basalts, olivine-clinopyroxenerich alkali basalts and trachybasalts; Nordeste ankaramites, relatively thick ankaramite flows; and upper basalts, olivine-alkali basalts, and aphyric trachybasalts and tristanites. The two youngest sequences, tristanites and trachytes, are limited to small flows and hypabyssal intrusions and to thin veneers of tophra. Numerous mafic dikes of varied composition and relative ages cut the complex.Feldspar compositions range from An86 to potash feldspar or Or56 Olivines range from Fo85 to Fo40, and clinopyroxenes range from diopsidic-augitcs to aluminous titanaugites to aegirine augites. Based on the Al-Ti content of the clinopyroxenes, two trends are recognized: high silica basalts (transitional) with clinopyroxenes enriched in Si, Ca, and Cr and a low-silica alkaline trend (ankaramite trachyte) with clinopyroxenes enriched in Al and Ti. Coexisting titanomagnetites and ilmenites indicate consolidation temperatures from 958 °C to 695 °C-and oxygen fugacites between 10-12.1 to 10-14.2.Plots of 60 chemical analyses on magnesia and alkali-silica variation diagrams show the bulk of the Nordeste volcanics to be a potassic, mildly alkaline, undersaturated to saturated, alkali basalt trachybasalt tristanite trachyte series. The oldest flows of the complex, characterized by low TiO2, high SiO2, and clinopyroxenes poor in TiO2 and Al2O3, are interpreted as representative of chemistries transitional between tholeiitic and alkalic suites. Comparison of the Nordeste volcanics and Terceira (Azores) volcanics show the latter to be more undersaturated and sodic in lieu of potassic. Compared to other Atlantic islands, Sao Miguel is most similar to Gough Island, differing in having higher Ti, K, Fe, and P, but lower Na and Mg, and late differentiates which are characteristically quartz normative.The chemical characteristics of the Nordeste Complex, as interpreted from phase chemistry and magnesia variation diagrams, and tested using the Wright and Doherty (1970) least-squares computer method for liquid lines of descent, support a crystal fractionation model. The magnesian (> 7.0% MgO) porphyritic-ankaramitic alkali basalts are accumulative in origin. The trachybasalts trachyte series represent fractionated liquids generated by the removal of titanaugite, plagioclase, and olivine in the early stages, followed by abundant plagioclase, augite, and minor opaques in the late stages, from “parental” alkali olivine basalt melts in crustal reservoirs (5 to 10 km). The predominance of plagioclase in the fractionation scheme and its rarity as an xenocrystic phase suggest the Azores platform to be underlain by gabbroic and dioritic layered complexes. 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.

Anomalous paleomagnetic directions in late tertiary andesitic intrusions of the Cauca depression, Colombian Andes

Late Tertiary hypabyssal intrusions of the northern Cauca depression in the northwest Andes of Colombia provide anomalous paleomagnetic declinations of approximately northwest trend. Individual tectonic rotation mechanisms of folding or faulting alone are incapable of accounting for the observed dispersed inclinations in the northwestern trend. Combinations of mechanisms, including regional shear, folding, and thrusting, may explain the anomalous paleomagnetic vector directions.

Geology and petrology of the Nordeste volcanic complex, São Miguel, Azores: Summary

The Nordeste volcanic complex of Sao Miguel island is one of the oldest known complexes (4.01 to 0.95 m.y.) of alkaline volcanism in the eastern Azores. It contains about 1,300 m of exposed volcanics and hypabyssal intrusives that have been subdivided into five stratigraphic sequences: lower basalts, Nordeste ankaramites, upper basalts, tristanites, and trachytes. The bulk of the complex is predominantly of basaltic composition and is confined to the three lowermost sequences: lower basalts, olivine-clinopyroxenerich alkali basalts and trachybasalts; Nordeste ankaramites, relatively thick ankaramite flows; and upper basalts, olivine-alkali basalts, and aphyric trachybasalts and tristanites. The two youngest sequences, tristanites and trachytes, are limited to small flows and hypabyssal intrusions and to thin veneers of tophra. Numerous mafic dikes of varied composition and relative ages cut the complex. Feldspar compositions range from An 86 to potash feldspar or Or 56 Olivines range from Fo 85 to Fo 40 , and clinopyroxenes range from diopsidic-augitcs to aluminous titanaugites to aegirine augites. Based on the Al-Ti content of the clinopyroxenes, two trends are recognized: high silica basalts (transitional) with clinopyroxenes enriched in Si, Ca, and Cr and a low-silica alkaline trend (ankaramite trachyte) with clinopyroxenes enriched in Al and Ti. Coexisting titanomagnetites and ilmenites indicate consolidation temperatures from 958 °C to 695 °C-and oxygen fugacites between 10 -12.1 to 10 -14.2. Plots of 60 chemical analyses on magnesia and alkali-silica variation diagrams show the bulk of the Nordeste volcanics to be a potassic, mildly alkaline, undersaturated to saturated, alkali basalt trachybasalt tristanite trachyte series. The oldest flows of the complex, characterized by low TiO 2 , high SiO 2 , and clinopyroxenes poor in TiO 2 and Al 2 O 3 , are interpreted as representative of chemistries transitional between tholeiitic and alkalic suites. Comparison of the Nordeste volcanics and Terceira (Azores) volcanics show the latter to be more undersaturated and sodic in lieu of potassic. Compared to other Atlantic islands, Sao Miguel is most similar to Gough Island, differing in having higher Ti, K, Fe, and P, but lower Na and Mg, and late differentiates which are characteristically quartz normative. The chemical characteristics of the Nordeste Complex, as interpreted from phase chemistry and magnesia variation diagrams, and tested using the Wright and Doherty (1970) least-squares computer method for liquid lines of descent, support a crystal fractionation model. The magnesian (> 7.0% MgO) porphyritic-ankaramitic alkali basalts are accumulative in origin. The trachybasalts trachyte series represent fractionated liquids generated by the removal of titanaugite, plagioclase, and olivine in the early stages, followed by abundant plagioclase, augite, and minor opaques in the late stages, from “parental” alkali olivine basalt melts in crustal reservoirs (5 to 10 km). The predominance of plagioclase in the fractionation scheme and its rarity as an xenocrystic phase suggest the Azores platform to be underlain by gabbroic and dioritic layered complexes.

The aeromagnetic mineralogy of igneous rocks

Magnetic minerals in igneous rocks are dominated by phases in the system FeO–Fe2O3–TiO2 and the system Fe–Ni–S. Complex solid solutions within both systems yield divergent magnetic signatures, which are dependent on primary mineralogy, on deuteric cooling, and on post-depositional alteration of the host rock. Intrinsic parameters such as temperature, oxygen fugacity, and sulfur fugacity, have effective buffering capacities, which influence the ratios Fe2+:Fe3+; (Fe2+ + Fe3+):Ti; Fe:S; and (Fe + Ni):S. These ratios are highly sensitive to environmental conditions and correlate with magnetic property measurements. In addition, bulk chemistry plays a major role, and for igneous rocks a striking pattern emerges in which oxygen fugacity is related to SiO2 (i.e., from basic to acid suites) regardless of total pressure. Acidic suites (granites and rhyolites) are more highly oxidized than basic suites (gabbros and basalts); hence. ulvöspinel (Fe2TiO4)-rich and ilmenite (FeTiO3)-rich constituents are characteristic of the latter, whereas magnetite (Fe3O4)-rich and hematite (αFe2O3)-rich components are typical of the former as primary precipitated oxides. Deep-seated equilibration results in exsolution and in low-intensity states of oxidation, and this is in contrast to extrusive suites in which intense states of high temperature oxidation are prevalent. Olivine decomposition in extrusive rocks, and the formation of magnetite solid-solution members in plagioclase and pyroxene in plutonic rocks, yield single domain particles, which in some instances are the dominant magnetic carriers. Sulfide minerals are rare in continental extrusive suites, are more abundant in oceanic basalts and in hypabyssal minor intrusives, and peak in plutonic environments.Within the context of deep crustal magnetic anomalies, and in the determination of Curie isotherms, it is proposed that the primary and secondary mineralogy of plutonic ultramafic rocks be carefully re-evaluated based on current models of serpentinization. Metallic iron and metal alloys of Fe–Ni–Co–Cu are widely recognized in these rock types, and because these minerals have elevated Curie temperatures (620–1100 °C) the depth detection limits of aeromagnetic anomalies need not be constrained by the accepted value of 580 °C, the Curie temperature of magnetite.

Magnetite-apatite-amphibole-uranium and silver-arsenide mineralizations in lower Proterozoic igneous rocks, East Arm, Great Slave Lake, Canada

The East Arm of Great Slave Lake contains a great thickness of Aphebian (2,460-1,750 m.y.) sedimentary and igneous rocks that are little deformed or metamorphosed. Three separate periods of intrusive magmatic activity have been recognized: (1) early Aphebian ( approximately 2,100 m.y.) alkaline complexes and probably related breccia pipes; (2) middle Aphebian volcanism ( approximately 1,870 m.y.) with hypabyssal intrusions; and (3) late Aphebian ( approximately 1,790 m.y.) calc-alkaline diorite stocks and laccoliths. These intrusive rocks all contain examples of the Ag, Ni-Co arsenide ore-type and the later ones particularly contain magnetite-apatite-amphibole with uranium and rare earth element minerals. A model for the genesis of the arsenide ore fluid is postulated from the study of one of the early Aphebian complexes and involves essentially hydrothermal leaching principally of mafic minerals. Models for various modes of deposition are presented and principally involve fluid migration up early joints and deposition in open spaces. The magnetite-apatite-amphibole deposits were emplaced earlier in joints, sometimes passively and sometimes forcefully, and were probably generated as immiscible liquids.The similarity of the ore deposits in the rocks of different ages suggests a common petrochemical process for their formation. The metallogeny of the late Aphebian Great Bear batholith is also closely similar which justifies the assumption of some geotectonic relationship. It has been proposed that the two ore associations may serve as geotectonic fingerprints for ancient subduction zones. These data from the East Arm add no definite evidence for or against the hypothesis but appear only to confuse the issue.

The Harp dikes and their relationship to the Helikian geological record in central Labrador

The Harp olivine diabase dikes, of Neohelikian age, form an east-northeast trending swarm that cuts rocks of the Harp Lake Complex in central Labrador. The petrography, and rock and mineral chemistry of the dikes indicate that they are transitional in character between tholeiitic and alkalic compositions. The major element chemistry of the dikes is similar to basaltic magmas from other comparable continental settings and in particular closely resembles basalts and diabase sills of the Neohelikian Seal Lake Group.Correlation of the Seal Lake – Harp dikes magmatism is suggested with two other groups of hypabyssal intrusions of olivine gabbro east and southeast of the Seal Lake synclinorium (Michael gabbros and diabase dikes in the Mealy Mountains complex). All of this basic magmatism may have been related to a Neohelikian zone of continental rifting or incipient rifting. Intrusion and extrusion of basic magma under conditions indicative of crustal extension closely follows, or is associated with, uplift and erosion of anorogenic anorthosite–'granite' complexes in other places in the world and is inferred to be a consequence of a continuing evolving process of mantle–crust interactions; in Labrador, the process began in the Paleohelikian with intrusion of major anorthosite–adamellite complexes.

Crystallization and differentiation of internal zones of hypabyssal granite intrusions (Adun Chelon massif)

Crystallization and differentiation of internal zones of hypabyssal granite intrusions (Adun Chelon massif)

Distribution characteristics of beryllium in granitoids of hypabyssal intrusions (as in Golgotay massifs, eastern Transbaykal)

Distribution characteristics of beryllium in granitoids of hypabyssal intrusions (as in Golgotay massifs, eastern Transbaykal)

Observations on the Proterozoic Seton Formation, East Arm of Great Slave Lake, Northwest Territories

The Proterozoic (Aphebian) Seton Formation is shown to extend across almost the entire length of the East Arm structural subprovince of the Great Slave Lake region, Northwest Territories. Earlier described as greenstones or basalts and recently as an andesite–rhyolite suite, the volcanic rocks which characterize the Seton Formation are clearly of spilitic–keratophyric affinity. The formation is composed of a sequence of marine to subaerial, spilitic basalt flows, trachytic flows, quartz keratophyric–and spilitic–basic pyroclastics, volcanic sandstones, jasper, banded ironstones, and intercalated marine epiclastic sedimentary rocks. Small hypabyssal intrusions of albite granophyre, albite, and quartz porphyry represent minor subvolcanic phases. Petrographic descriptions of the lavas and pyroclastic rocks from Toopon Lake, the Fort Reliance area, and Seton Island are augmented by partial chemical analyses of 15 lavas from the latter locality. The volcanic–sedimentary Seton Formation, 1300 m thick in the SW of the East Arm, and 40 m thick in the Fort Reliance district, should be classified as a member of the Sosan Group, being in part laterally equivalent to the Akaitcho River Formation and the upper Kluziai Formation. The Aphebian Coronation Geosyncline during Seton times was thus characterized by effusive (partially submarine) island volcanism in the SW of the region, contemporaneous with shallow marine sedimentation towards the northeast part of the basin.

Tectonic Transport of Massive Sulfide Deposits in Submarine Volcanic and Sedimentary Host Rocks

Massive Cu-Zn-Fe sulfide deposits appear to be replacements or syngenetic sediments related to submarine volcanism. Undisturbed kuroko deposits or moderately deformed and metamorphosed orebodies like those of the Noranda district may be considered as models.Massive sulfide deposits in high grade metavolcanic and metasedimentary rocks are postulated to have originated largely in a submarine volcanic environment near the edges of technically active troughs sinking and receiving clastic sediments. Adjacent rising segments eventually expose deep structures almost devoid of base metals. Massive sulfides and volcanics, together with enclosing sediments, are susceptible to transportation by slides and nappes. Thus ores and enclosing rocks may be moved several kilometers and effectively separated from former root structures such as alteration zones and hypabyssal intrusives. The Elizabeth massive sulfide deposits, Vermont thus appear to have moved westward from root zones near the Connecticut River prior to their metamorphism. Possibly some of the metamorphosed massive sulfide deposits of the Sanbagawa terrane, Shikoku, Japan have been transported southward from root zones near the Median Tectonic Line. After initial transposition by sliding, ores and host rocks may be subjected to deep burial, strong metamorphism and one or more episodes of folding.

Metallogenic provinces and epochs in southern Central America

The southern part of Central America is characterized by common geological features. The ophiolitic belt running alongside the Pacific is related to the late Mesozoic eugeosynclinal trough. No significant mineralization is known in this rock complex. Copper ore occurrences in the Azuero peninsula are associated with the late Cretaceous diorite pluton. During Tertiary time, radial tectonic deformation and basaltandesite-dacite volcanism took place. Gold-silver quartz veins and Cu-, Pb-, Zn- (Ag)-sulfidic ore formation associated with this magmatic and tectonic activity form the largest metallogenic provinces. Porphyry copper mineralization in northern Panama is confined to the late Tertiary intermediate hypabyssal intrusives. Laterite, and gold placers are the only exogenic deposits. The relative abundance of copper ores in southern Central America indicates a geochemical specialization of magma in copper and the possible continuation of the circum-Pacific copper belt across Central America.

A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Eastern margin of the Red Sea and the coastal structures in Saudi Arabia

Results of many investigations since 1950 show that the eastern margin of the Red Sea and associated coastal structures in Saudi Arabia have a long geologic history, starting with the deposition of Precambrian eugeosynclinal sedimentary and volcanic rocks before 1000 Ma ago and extending to recent geologic time. The northeastern flank of the Red Sea rift valley is in a shield area affected by possibly four plutonic events at 1000, 720 to 735 (?), 660 to 670, and about 570 Ma. Cratonization of the shield occurred during all or part of the span 520 to 590 Ma. Nubian-type sandstone of Cambrian and Ordovician age laps up on the shield from Jordan southeastward around the rim of the Great Nafud basin, and along the eastern edge of the shield southeastward to 45° E longitude where it is overlapped by Permian limestone. The sandstone reappears to the south and extends southward and westward to the Asir Mountains at the Yemen border. Isolated sandstone outliers are present in the central shield, proving that lower Paleozoic sandstone covered most, if not all, of the basement as now exposed. The Mesozoic era was almost entirely a time of uplift and non-deposition except a middle to late Jurassic fringe marine invasion in the south and a possible narrow invasion from the Gulf of Suez at the end of the era. Marine and non-marine sedimentary deposits of middle and late Tertiary age are found along the Red Sea coast, and Oligocene basaltic flows are present at both low and high altitudes in the coastal ranges. Evidence for important volcanism during Oligocene and earliest Miocene time is widespread, and within the eastern rift fault zone early Miocene hypabyssal intrusives cut the shear zones. Major rifting occurred just before or during early Miocene when the flanks of the rift valley were ramped upward. Shortly after this volcanism a thickness on the order of 3500 m of middle Miocene marl and evaporite beds filled the Red Sea trough. Evidence also exists for widespread subaerial erosion in the Pliocene. Younger lava flows are Pliocene in age but the youngest, near Al Medinah, came as late as A. D. 1250. Lake-bed deposits are very probably in large part Pliocene throughout the shield. The Red Sea coastal plain in Saudi Arabia rises gently eastward from a 3 m littoral surface, generally underlain by dead reef from the Yemen border northward to Al Wajd, a distance of 1400 km. At Jizan, in the south, a salt dome has pushed the 3 m surface up to an elevation of about 50 m. From Al Wajd northward, Pleistocene terraces have been faulted, culminating in several surfaces as high as 520 m above the Red Sea at Tiran Island. Ramping of major fault-bounded blocks along the eastern side of the Red Sea trough—the Midian block in the north, a poorly defined central block, and the Asir block in the south—is connected with renewed movement on regional Precambrian faults. Drainage patterns of wadis in these blocks are characteristically affected by the ramping, and stream capture is common in the Midian and Asir blocks.

Undersaturated Potassic Lavas and Hypabyssal Intrusives in North Johore

AbstractAn interesting occurrence of highly potassic lavas and hypabyssal intrusives of assumed Tertiary age is described from North Johore. The thickest lava flow is gravitatively differentiated showing a basal horizon of potassic ankaramite and basanite grading up into highly potassic bolitized flow tops of leucite-tephrite composition. Evidently the parent magma was close to a biotite pyroxenite in composition and possibly an assimilation product of peridotite by granite.

PETROGRAPHIC AND STRUCTURAL ASPECTS OF THE NORTH CAUCASUS AMPHIBOLE ASBESTOS IN THE KHATSAVIT DEPOSIT

Deposits of fibrous amphiboles which differ from previously known occurrances in the Central Caucasus in being of distinctive compositions and geologic positions. The deposit is in the top of an anticline of Devonian sediment volcanic strata. It is related to hydrothermal veins in metasomatized gabbroids. Small hypabyssal intrusions of granite-porphyry are structurally related to the asbestos veins. The petrography of the deposit and the related rocks is described in detail, as is the structure. Four generations of mineralization are recognized. Alteration shows a pronounced zonal distribution. The outer area of the zoning is well defined because of regional metamorphism. The asbestos is the result of interaction between the hydrothermal fluids and the country rock. Structurally a combination of folding with NE trending shears and NW trending tension fractures localize the ore deposits. The fibrosity of the actinolite is related to wild movement during tectonism. M. A. Klugman

Fluorite-bastnaesite deposits of the Gallinas Mountains, New Mexico, and bastnaesite paragenesis

Many early reports on the occurrence of bastnaesite indicate that the mineral crystallized under a high-temperature environment. The Gallinas Mountain (New Mexico) bastnaesite deposits, however, occur as epithermal veins and breccia fillings in unaltered Permian sandstone in proximity to early Tertiary alkalic hypabyssal intrusives. The low temperature of formation is indicated by the predominance of open-space fillings, little replacement, extensive crustification, and lack of wall rock alteration. In addition, a geothermometric fluid inclusion study indicated a temperature of formation of 175-185 degrees C.This re-examination of a previously-known deposit and a study of other bastnaesite occurrences shows that the mineral is generally indicative of mineralization of alkalic derivation. Bastnaesite, however, is not a critical mineral diagnostic of any particular geological environment or temperature of formation. Bastnaesite occurs in magmatic, hydrothermal, contact metasomatic, and carbonatite deposits. It is both a primary and secondary mineral.

TECTONIC MOVEMENT AND ROCK STRUCTURE OF THE SAMBAGAWA METAMORPHIC ZONE, JAPAN

The Sambagawa crystalline schist was derived from a Carboniferous-Permian rock suite by later metamorphism. The Kino-kawa (river) basin, having being separated by an uplift in the geosynclinal stage into a northern and southern region subsided on both sides and received depositions of basic volcanic ejecta. The northern area of subsidence, continuous as far as central Shikoku, did not subside uniformly, but a southward projecting part subsided deeper and received thicker deposits of ejecta than the rest. In the northern region of subsidence transgressive sedimentation resulted in imbricate structure from north to south. The central uplift area consisted of argillaceous and arenaceous shales with some thin beds of volcanic ejecta. While the orogenic movement was in progress, both basic and ultrabasic hypabyssal intrusions occurred in the subsided areas. Those intrusions were accompanied by conversion from geosynclinal development to geanticlinal development. As the result of the orogenic movement, gentle s...

Further Palaeomagnetic Results from South Victoria Land, Antarctica

A palaeomagnetic study has been made of rock samples taken from a 5000 ft sequence in South Victoria Land. The sequence consists of granites, sediments and hypabyssal intrusives of Mesozoic, Palaeozoic and possibly Pre-Cambrian ages. The directions of natural remanent magnetization are approximately uniform throughout, but small and apparently real differences do occur between some of the units within the sequence. Laboratory tests using alternating magnetic fields suggest that these differences are due to the presence of unstable components of small but variable magnitude and that the mean directions of the stable components are parallel in all the units studied. This uniformity in direction could have resulted from the geomagnetic field in the region being constant in direction for a long period of time in the Palaeozoic and Mesozoic, or from the reheating of the whole area during the last phase of intrusion (that of the Ferrar dolerites) in Mesozoic times. The latter interpretation is favoured. The variations in magnetic properties through the Ferrar dolerite sheets are described and provide information relevant to theories of their emplacement and differentiation.

Structural significance of Tertiary volcanic rocks in southwestern Utah

Most of the silicic volcanic rocks of the Great Basin are ignimbrites rather than lava flows. The more common types were probably formed by nuees ardentes which spread laterally as density currents of very high fluidity, but there are aberrant types that may be in some manner transitional between ignimbrites and lava flows, and the whole problem of mechanism of origin is as yet little understood. Uncertainty on this score is of no concern here; mapping of many of the units indicates that, whatever their origin, they assumed a distribution approaching that of an equal volume of water, filling valleys to a common level and forming sheets of substantially uniform thickness where the relief of the pre-existing surface was low. Some of the individual ignimbrites are many hundreds of feet thick and have an areal extent of as much as 10,000 sq. mi. Because each of these extensive and initially flat sheets was formed everywhere at the same instant of time, they are very nearly ideal stratigraphic units. They occur in most parts of the Great Basin and range in age through most of the Tertiary. Eleven ignimbrites which are widespread in southeastern Utah are described and given formal stratigraphic names. The fact that the oldest of them lies unconformably across the beveled edges of thrusts and folds involving late Cretaceous strata indicates that the beginning of volcanic activity postdates the Laramide orogeny. As planar units which provide a record of Tertiary crustal movements, the ignimbrites confirm the Gilbert theory, based originally on physiographic evidence, that block faulting has been the characteristic type of post-Laramide deformation in the Great Basin. The stratigraphic-structural approach makes it possible 1) to work out the geometry of the block faulting with a precision not obtainable by use of displaced erosion surfaces; 2) to deal with episodes of block faulting that occurred during the early Tertiary and are not expressed by the present topography; and 3) to crossdate these and other geologic events on a regional scale. On the basis of detailed work in southwestern Utah, and reconnaissance elsewhere, it is stated as a deliberately provocative working hypothesis that block faulting has been the only type of regional tectonism in the Great Basin in postorogenic time. The first requirement in testing this hypothesis is the recognition, as such, of flexures and thrust faults developed 1) by emplacement of hypabyssal intrusions, and 2) by gravity sliding from primary relief features raised by intrusion and block faulting. These 2 classes of structures are of much interest in their own right; their significance for present purposes is merely that they may be readily mistaken as evidences of regional tectonism. Equally critical is the need for distinguishing postorogenic deformational effects from those produced during the orogeny. Examples are given of use of the ignimbrites in making these distinctions.

Contact Metamorphism at Maddocks Hill near Wellington, Shropshire

AbstractA large basic sill intrudes the Shineton shale (Cambrian) at Maddocks Hill, east of the Wrekin, near Wellington, Shropshire. An old quarry exposes the contact and reveals that the shale close by has been recrystallized to a hard, compact hornfels, the intensity of alteration decreasing away from the contact. Thin sections show that the metamorphic effect was solely thermal, the rock having been completely recrystallized, and biotite having been generated under conditions of the amphibolite facies. The metamorphism is compared with that brought about by other basic hypabyssal intrusions.

Igneous rocks of the Terlingua-Solitario region, Texas

The Terlingua-Solitario region in southern Brewster and Presidio counties, Texas, is arid and mountainous with many hypabyssal intrusions. Lava flows border it on the west. The sedimentary rocks, excepting some of Paleozoic age in the center of the Solitario, the largest laccolithic dome, are Cretaceous and have been deformed by laccolithic intrusions and faults, many of the rift type. The Solitario contains many small rhyolite dikes and sills and is encircled by a rim sill. Volcanic agglomerate in a central cauldronlike area records an extrusion. The eastern part of the area is in a block depressed several thousand feet and contains many dikes, sills, and smaller laccoliths. The igneous rocks include an analcite-bearing series and intermediate, trachytic, and rhyolitic types, most of which are soda-rich. The analcite-bearing types range from melanocratic gabbroid to syenitic types. Analcite is primary, deuteric, and hydrothermal and was formed through a rather long period. The complete assemblage is alkaline and resembles the suites from Spanish Peaks, Colorado, and other alkaline subprovinces along the front of the Rocky Mountains. The analcite-bearing rocks are most abundant in the sunken block and evidently were derived from several differentiating submagmas. The other rocks, especially the rhyolites, are associated with higher anticlinal structures. A tectonic control of differentiation is postulated.