Younger Granitic Rocks Research Articles

Geochemical constraints and uranium potential of the younger granitic rocks in El Maghrbia area, Central Eastern Desert, Egypt

Geochemical constraints and uranium potential of the younger granitic rocks in El Maghrbia area, Central Eastern Desert, Egypt

Tectonic evolution of the Neoarchean Mundo Novo greenstone belt, eastern São Francisco Craton, NE Brazil: Petrology, U-Pb geochronology, and Nd and Sr isotopic constraints

Field, petrographic, lithogeochemical, LA-ICP-MS in situ U-Pb zircon geochronology, and Nd and Sr isotopic data were used to determine the geologic evolution, timing of volcanism and tectonic setting of the Mundo Novo greenstone belt (MNGB) including nearby units, such as the basement and younger granitic rocks in the eastern São Francisco Craton (NE Brazil). The basement of the greenstone belt corresponds to the Gavião Block, which comprises tonalite-trondhjemite-granodiorite orthogneisses, migmatites and subordinate metarhyolites and metagranites. The MNGB is divided into three lithological sequences: (i) the lowermost unit is composed of metakomatiites; (ii) the middle unit is composed mainly of metabasalts, metadacites and metasedimentary chemical rocks; and (iii) the uppermost unit is composed of metasedimentary siliciclastic rocks. Additionally, two Rhyacian-Orosirian granitic plutons occur in the area, the Areia Branca and Jequitibá metagranites. U-Pb zircon data from the Miguel Calmon and Fazenda Coqueiro metagranites in the Gavião Block yield crystallization ages of 3355 ± 16 Ma and 3227 ± 23 Ma, respectively, and both metagranites were derived from the lower continental crust based on low 87Sr/86Sri values (0.699 and 0.705, respectively) and negative values of ƐNd(t) (−3.7 and −3.6, respectively). The 2595 ± 21 Ma U-Pb zircon crystallization age of the metadacites allowed the determination of the timing of volcanism in the middle sequence of the MNGB. The 2106 ± 71 Ma and 1975 ± 36 Ma U-Pb zircon crystallization ages for the Areia Branca and Jequitibá metagranites, respectively, which are both interpreted as derived from crustal protoliths based on high 87Sr/86Sri values (0.744 and 0.730, respectively) and negative values of ƐNd(t) (−7.7 and −6.5, respectively), record a Rhyacian-Orosirian tectonothermal event that affected the MNGB and the adjoining region. The whole-rock geochemical data in tectonic discrimination and multielement diagrams show that the volcanic rocks of the MNGB formed in island arc and back-arc basin settings related to the oceanic crust. Therefore, the following five stages are proposed for the tectonic evolution of the study region: stages (i) and (ii) were characterized by the establishment of the TTG basement by 3.35 Ga, when the Miguel Calmon granite was consolidated, followed by a rifting that formed the rhyolites, and subsequent granite formations occurred at 3.22 Ga (Fazenda Coqueiro granite formation), followed by the abort of the rift and a closure event. The volcanism during stage (iii) at 2.59 Ga in the MNGB occurred in island arc and back-arc basins in the oceanic crust between Archean cratonic blocks, and stages (iv) and (v) between 2.10 Ga and 1.97 Ga represent two stages of a progressive collision event in which the oceanic crust was compressed between the Gavião, Mairi, Jequié and Serrinha blocks and the Itabuna-Salvador-Curaçá Belt from the eastern part of the São Francisco Craton. The collisional event of stages (iv) and (v) was coeval with the Rhyacian-Orosirian formation of granites and basins such as that where the uppermost siliciclastic sequence of the MNGB was deposited.

Curie depth and geothermal gradient from spectral analysis of aeromagnetic data over Upper Anambra and Lower Benue Basin, Nigeria.

The resent (2009) aeromagnetic data covering lower part of Benue and upper part of Anambra basins was subjected to one dimensional spectral analysis with the aim of estimating the curie depth and subsequently evaluating both the geothermal gradient and heat flow for the area. Curie point depth estimate obtained were in the range of 25 km to a maximum of 32 km, the maximum values where obtained within the regions of positive magnetic anomalies. The geothermal gradient within this area varies between 320C/km to 800C/km. The highest geothermal gradient is observed around Katakwa at the northern edge, which host the young granitic rocks of central Nigeria and around Lokoja which host undifferentiated old granites of western Nigeria. Heat flow values obtained are between 46mW/m2 and 98 mW/m2. Shallow Curie point depths, high geothermal gradient and high heat flow, located at two geometric basement highs at the western and northern parts, correlate with regions with high concentration of both potassium and Thorium concentrations as observed on the ternary map.Keywords:- Geothermal Gradient, International Geomagnetic Reference Field (IGRF), Benue Trough

Field Occurrence and Petrographic Characteristics of Tertiary Volcanic Rocks and Associated Intrusions in and around Taiz City, Yemen

Geologic and petrographic studies of Tertiary volcanic rocks and their intrusive bodies such as plutons, dykes and sills were carried out in and around Taiz city, Yemen with an aim to document their field occurrence and distribution as well as to study their mineralogical composition. This has an important bearing on the town planning in the city of Taiz as the existing buildings are collapsing due to the foundation problems. Studies on 110 exposures, carefully selected after field traverses revealed that the Tertiary rocks in the study region are mainly represented by typical bimodal mafic-felsic associations in the form of flows, plutons and dykes. Tertiary volcanic flows are characterized by alternating sequences of basic and felsic rocks and varicoloured volcaniclastic deposits which were all extruded and fed from fracturing and fissuring in the old rocks through which magma emerged in successive pulses, flooding the surrounding region. The basic flows (Tb1, Tb2 and Tb3) consist of jointed/massive basaltic rocks and volcaniclastic deposits while the felsic flows (Tr1 and Tr2) are comprised of jointed/massive rhyolite/dacite rocks and also varicoloured felsic volcaniclastic deposits. All these rocks show wide variations in their geological and petrographical characteristics such as colour, texture, heterogeneity, macro/microfractures, weathering/alteration, thickness, horizontal attitude, and intercalation with volcaniclastic deposits, repetition with depth in both vertical and horizontal directions. Volcaniclastic deposits are also characterized with great diversity in their types, colours, textural features, thicknesses, grain sizes, matrices, and degree of roundness of rock fragments and alternating and/or interlocking as well as intercalation laterally and vertically with basalt/rhyolite lava rocks. Volcaniclastic rocks were classified for the first time in the study area based on their particle sizes into different types. They range from strong, compact, welded rocks to weak, altered soils. The younger Sabir granitic pluton is represented by alkaline or peralkaline granites that are white to greyish white coloured massive, medium to coarse-grained, grading up to granite porphyry. Petrographic examinations of 52 thin sections representing the samples of basaltic lava flows, rhyolitic/dacite lava flows and younger intrusives under polarizing microscope have been carried to study their mineralogy. The dominant minerals in basaltic rocks are plagioclase, augite and olivine whereas rhyolitic rocks contain quartz, orthoclase and biotite. The main mineral constituents of younger granitic rocks are k-feldspar, quartz, hornblende and biotite. The mafic and felsic dykes almost resemble basalts and rhyolites/dacites respectively in their petrographical characteristics. XRD technique used for characterizing the minerals of volcanic soils revealed the presence of clay minerals namely montmorillonite and kaolinite as the major mineral phase in most of the samples whereas mica group of minerals such as muscovite, vermiculite and chlorite are present in minor amounts, in addition to talc, feldspar, calcite and halloysite.

Magnetic Studies of Kagoro and Environs, North Central Nigeria

Magnetic data were compiled from four aeromagnetic maps. A submap for the study area was produced from the four magnetic sheets in an ILWIS environment. Magnetic intensity values vary between 32,600 and 33,160 nT. The lowest magnetic contour values closure is centred over the Kagoro granite rocks at the central part. This closure is structurally aligned along the NE - SW direction. Regional field values vary between7789.1 and 7896.6 mGal. The residual contour values range from -180 to +160 nT. The main closure of the residual map is located on the central part of the area. A slipper-shaded closure centred over the Kagoro Granite is oriented along the NNE - SSW direction. Some of the closures in the study area are aligned along the NE - SW and E - W directions. Patterns of the closures of the magnetic maps are in conformity with those of the Nigeria basement and younger granite rocks. Isolations and magnitudes of the closures suggest that the various rock bodies occur separately and at different levels. The regional fields along nine profiles were obtained by third degree polynomial fitting using OriginLab7 software so as to obtain the residuals used for the models. The residual anomalies were accounted for by Mag2dc (2.5D) algorithms. The four profiles that traversed the Kagoro younger granite complex were estimated at maximum thicknesses of 14.1, 15.2, 9.5 and 10.4 km. These suggest that the Kagoro younger granite complex was emplaced at depths between 9.5 and 15.2 km. The volcanic rocks and their associates were formed from depths between 4.7 and 13.0 km.

Trace Elements and Health: An Environmental Risk in Nigeria

The concentration levels of trace elements in drinking water and food pose potential health risks to man and therefore require great attention. Studies on iodine deficiency disorders (IDD) indicate goiter prevalence in areas underlain by metamorphic Basement and Younger Granite rocks and its near absence in the sedimentary terrains. There are cases of dental fluorosis resulting from the excessive ingestion of fluoride in both children and adults living in both the crystalline and sedimentary terrains in Nigeria with none reported from the coastal areas. Mining/mineral processing predisposes these trace elements to weathering whereby they are released into the environment in the soil and water bodies. The incidence of abnormally high natural radiation (radon gas) associated with most rocks and the exceptionally higher natural radiation associated with cassiterite mill tailings of the Jos Plateau, north central Nigeria, is a source of worry. Epidemiological records have indicated the increasing rate of lung cancer prevalence on the Jos Plateau and studies carried out elsewhere in Europe have linked about 13% of deaths associated with lung cancer to exposure to natural radiation. Preliminary hydro-geochemical study indicates high concentrations of potentially harmful elements (PHE) (Pb, Cu, Zn, As, Cd, Hg, V, Co, Cr, Ni, etc.) in the soils and water bodies in the crystalline and inland sedimentary terrains especially close to sulfide mineralization. In addition, high levels of these elements from anthropogenic sources have also been reported in soils and water in most mining/mineral processing sites and urban centres. The knowledge of the relationships between trace elements and human health issues is at its infancy in Nigeria. To adequately understand these relationships, there is need for geoscientists to intensify research on trace elements in the environment and together with professionals in community health so as to identify health issues arising from trace elements in the environment.

A Detailed Geochemical Study of a Shallow Arc-related Laccolith; the Torres del Paine Mafic Complex (Patagonia)

The shallow crustal Torres del Paine Intrusive Complex in southern Patagonia offers an opportunity to understand the chemical evolution and timing of crystallization processes in shallow plutonic rocks. It is characterized by hornblende-gabbros, gabbronorites, monzodiorites and granitic plutonic rocks.The exceptional exposure of the intrusion permits the identification of two structurally and petrographically different zones. Layered gabbronorite, olivine-bearing pyroxene-hornblende gabbronorite and monzodiorite forming vertical sheets and stocks in the west are referred to here as the feeder zone. These mafic rocks are in vertical contact with younger granitic rocks on their eastern border. The eastern part is a laccolith complex. It is characterized by three major units (I, II, III) of granitic rocks of over 1000 m vertical thickness; these are underlain in places by a sequence of hornblende-gabbro sills intermingled with evolved monzodiorite granite. Chilled, crenulated margins as well as flame structures between gabbroic rocks and monzodiorites suggest that the mafic sill complex remained partially molten during most of its construction. Bulk-rock major and trace element data indicate that the Paine mafic rocks follow a high-K calc-alkaline to shoshonitic differentiation trend. The parental magmas were basaltic trachyandesite liquids, with variable H2O and alkali contents.The majority of the feeder zone gabbronorites have high Al2O3 contents and positive Eu and Sr anomalies, consistent with accumulation of plagioclase and efficient extraction of intercumulus melt.The mafic sill complex largely lacks these cumulate signatures. Comparisons of the intercumulus groundmass in the hornblende-gabbros with intra-sill dioritic stocks and pods reveal similar rare earth element patterns and trace element ratios indicating incomplete extraction of evolved interstitial liquids.The Sr, Nd and Pb isotopic compositions of the mafic and granitic rocks exhibit ranges of 87Sr/86Sr of 0·704-0·708, eNd +3·8 to -1.2, 206Pb/ 204Pb 18·61-18·77, 207Pb/ 204Pb 15·67-15·67 and 208Pb/ 204Pb 38·56-38·77. Crystal fractionation and assimilation-fractional crystallization modelling, combined with high-precision U-Pb dating of zircons, indicates that the western feeder zone gabbronorites are linked to the uppermost Paine granite (granite I), whereas the mafic sill complex is younger and not directly related to the voluminous granite units II and III. These results are interpreted to indicate that crystal-liquid separation is facilitated in subvertical, dynamic feeder systems whereas subhorizontal sill complexes are inefficient in separating large volumes of mafic cumulates and complementary felsic rocks.

Utilities of Landsat 7 data and selective image processing in characterization of radioactivity zones of Wadi Baba–Wadi Shalal Area, Westcentral Sinai, Egypt

Wadi Baba–Wadi Shalal area locates in the westcentral part of Sinai, Egypt. It is covered by a Precambrian basement rocks comprise the northern part of the Precambrian Arabo-Nubian crystalline massive. The lithologic and structural setting of the investigated area was interpreted from the digital Landsat-7 Enhanced Thematic Mapper Plus (ETM+) data. The structural lineament analyses for the lithologic units and their relationships to the high-radioactivity zones (HRZ) and the characterizations of HRZ of the study area are the main tasks of this article. Extraction algorithm was applied using Geomatica PCI package under the user defined parameters. The extracted structural lineaments have been evaluated and chicked using the visual interpretation and published works. The short-wave infrared spectral ETM+ band-7 was selected as an optimum data for automatic lineaments extraction since it scored the highest lineament frequency (1856) compared to the other visible and near infrared bands. The aeroradiometric color raster total count equivalent thorium (eTh) and equivalent uranium (eU) maps were used to delineate the highest radioactivity zones of the study area. A selective image processing technique (SIPT) is a new approach in Geomatica (9.1), which gives rise to valuable results in this work. The SIPT was carried out for the subset of the ETM+ data of the highest radioactivity zones. The spatial distribution of the structural lineament pattern maps for some low-radioactivity zones (LRZ) and for the highest radioactivity zones of the study area are prepared with their frequency rose diagrams. The NE–SW trend is the predominant structural lineaments trend in the investigated area. The NE–SW to the ENE–WSW directions are the predominant structural lineament trends in both the LRZ and the HRZ. These high-radioactivity zones of the investigated area are characterized by high lineaments density and lineaments-intersection density, restricted to Um Bogma Formation and younger granitic rocks and are not controlled by structural lineament trends.

Stratigraphy, facies architecture, and palaeoenvironment of Neoproterozoic volcanics and volcaniclastic deposits in Fatira area, Central Eastern Desert, Egypt

Fatira area in the Central Eastern Desert, Egypt, is a composite terrane consisting of Neoproterozoic volcanics and sediments laid down in submarine to subaerial environment, intruded by voluminous old to young granitic rocks. The various lithofacies of the study area can be grouped in three distinct lithostratigraphic sequences, which are described here in stratigraphic order, from base to top as the Fatira El Beida, Fatira El Zarqa and Gabal Fatira sequences. Each depositional sequence, is intimately related to volcanic activity separated by time intervals of volcanic inactivity, such as marked hiatuses, reworked volcaniclasts, and or turbidite sedimentation. Four submarine facies groups have been recognized within the oldest, folded eruption sequence of Fatira El Beida. The southern part of the study area is occupied by sheet lava (SL), pillow lavas (PL), pillow breccias (PB), and overlying Bouma turbiditic volcaniclastites (VC). The four facies groups of Fatira El Beida sequence occur in a predictable upward-deepening succession, essentially from base to top, an SL–PL–PB–VC stacking pattern. The coeval tholeiitic mafic and felsic volcaniclastic rocks of this sequence indicate an extensional back-arc tectonic setting. The El Beida depositional sequence appears to fit a submarine-fan and slope-apron environment in an intra-arc site. The Fatira El Zarqa sequence involves a large volume of subaerial calc-alkaline intermediate to felsic volcanics and an unconformably overlying siliciclastic succession comprising clast-supported conglomerates (Gm), massive sandstone sheet floods (Sm) and mudstones (FI), together with a lateritic argillite paleosol (P) top formed in an alluvial-fan system. The youngest rock of Gabal Fatira sequence comprises anorogenic trachydacites and rhyolites with locally emergent domes associated with autobrecciation and sill-dyke rock swarms that could be interpreted as feeders and subvolcanic intrusions. Unconformity and lithofacies assemblages define seven events and three unconformity-bounded tectonic stages that record uplift-subsidence cycles in the study area. A proximal–distal relationship has been established within the depositional products, based on the relative dominance of erosional and depositional features.

Fracture systems in the younger granite rocks around fobur, northern Nigeria: product of residual stress?

Outcrop-scale fractures and associated veins found in the Younger Granite rocks around Fobur, Northern Nigeria were studied in relation to similar structures in their host rocks (Migmatites-gneisses -quartzite complex). Fractures and veins attitude (strike and dip) data were collected across the study area and subjected to graphical and stereographic analyses. The result revealed a predominant N-S strike with variations in the NE-SW and NW-SE directions; and others trending in the E-W direction (dip joints). Field observations indicated neither deformation of joints planes nor distortion of joint directions which confirmed that the jointing occurred in an anorogenic setting. Nevertheless, the joint structural trend in the Younger Granite rocks replicated those found in the surrounding orogenic basement rock exposures. This could reflect the likely existence in the host Migmatite-gneiss-quartzite complex imposed residual stresses of the Pan African event that were not fully relieved. These ancient stress systems possibly helped to orient the fractures and other structures in a later emplaced Younger Granite complex in a fashion similar to the enclosing basement complex rocks.KEY WORDS: Residual Stress, Fractures, Younger Granites, Fobur, Nigeria

History of post-intrusive hydrothermal systems indicated by fluid inclusions occuring in the young granitic rocks at the Kakkonda and Nyuto geothermal systems

History of post-intrusive hydrothermal systems indicated by fluid inclusions occuring in the young granitic rocks at the Kakkonda and Nyuto geothermal systems

40Ar-39Ar and K-Ar geochronology for plutonic rocks in the central Abukuma Plateau, northeastern Japan

The Abukuma Plateau is one of the major granitic terranes in the Japan Arc. Older granitic rocks (foliated and intermediate), younger granitic rocks (massive and felsic), and minor gabbroic rocks are complexly distributed in this region. In order to examine the cooling histories of the plutons, we have determined four 40Ar-39Ar hornblende ages and five K-Ar biotite ages of the plutonic rocks in the central Abukuma Plateau. The results are as follows: Utsushiga-take gabbroic body: 103.8 ± 0.5 Ma (hbl) Nagaya body (older): 97.3 ± 0.6 Ma (hbl), 93.4 ± 3.1 Ma (bt) Shikayama body (older): 103.0 ± 0.4 Ma (hbl), 86.1 ± 3.8 Ma (bt) Ishimori body (older): 99.0 ± 0.5 Ma (hbl), 94.2 ± 2.6 Ma (bt) Miharu body (younger): 91.3 ± 2.3 Ma (bt) Hatsumori body (younger): 90.5 ± 2.6 Ma (bt) (hbl, 40Ar-39Ar hornblende age; bt, K-Ar biotite age).On the basis of these analytical results and the petrography of the samples, we made the following observations:(1) The cooling rates of the Nagaya and Ishimori bodies (56 and 46 °C/m.y.) are equivalent to the common cooling rates of the Japanese granitic batholiths. The cooling rate of the Shikayama body (13 °C/m.y.) is slow because its volume is large.(2) The excellent 40Ar-39Ar plateau age of and the scarcity of metamorphic minerals in the Utsushiga-take gabbroic body suggest that the hornblende age is not a rejuvenated age but the original cooling age.(3) We were unable to detect any distinct geochronological boundary between the older and younger granitic rocks in the central Abukuma Plateau, in terms of the K-Ar biotite ages.

Electron Microprobe Chemical Dating of Uraninite as a Reconnaissance Tool for Leucogranite Geochronology

AbstractWe suggest that electron microprobe techniques may be employed to date Tertiary samples of uraninite (UO~2~), which can contain very high concentrations of radiogenic Pb after only a few million of years of U and Th decay. Although uraninite is regarded as a rare accessory mineral, it is relatively abundant in leucogranitic rocks such as those found in the Himalayan orogen. We apply the U-Th-total Pb electron microprobe chemical dating method to a uraninite crystal from a ca. 18.3 Ma dike of the Mugu granite from the Upper Mustang region of central Nepal. With this technique, we calculate a mean chemical date that is consistent with isotope-dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb dates obtained from seven other uraninite grains and a monazite crystal from the same sample. Electron microprobe chemical dating yields results that typically will be an order of magnitude less precise than conventional dates: in the specific case of the Mugu granite, single point chemical dates each have ca. 1.5 Ma 2[sigma] (95%) confidence level uncertainties. However, the mean chemical date of 15 point analyses of the crystal we study has a 2SE (2 standard error) uncertainty of ca. 400 ka, comparable to uncertainties obtained with ID-TIMS. These results show that electron microprobe chemical dating of uraninite has substantial promise as a reconnaissance tool for the geochronology of young granitic rocks. The electron microprobe work also reveals substantial chemical complexity within uraninite that must be taken into account. The analyzed crystal displays a texturally and chemically distinctive core and rim that suggests episodic growth. Concentration gradients in U, Th, and Y across the boundary imply diffusive modification. We estimate the diffusivity of U, Th, and Y in uraninite at ca. 700 °C to be > 10-7 cm2 s-1. In contrast, Pb shows no distinctive concentration gradient across the core-rim boundary, implying that Pb has a much higher diffusivity in uraninite than U, Th, or Y. We estimate that Pb loss of as much as ca. 8.9% has occurred in the uraninite grains we analyzed by ID-TIMS.

Geology of the ignimbrites and the associated volcano–plutonic complex of the Ezine area, northwestern Anatolia

The Ezine region is located in the northwestern part of Anatolia where young granitic and volcanic rocks are widespread and show close spatial and temporal association. In this region magmatism began with the Kestanbol granite, which intruded into metamorphic basement rocks, and formed contact metamorphic aureole. To the east and southeast the pluton is surrounded by hypabyssal rocks, which in turn, are surrounded by volcanic associations. The volcanic rocks may be divided into two main groups on the basis of their lithological properties. Lavas and lahar deposits dominate the northern sector while ignimbrites dominate the southern sector. The ignimbrite eruptions were formed partly coevally with the plutonic and the associated volcanic rocks during the early Miocene. They appear to have been associated in a caldera collapse environment. Geochemical properties of the plutonic and the associated volcanic assemblages indicate that the magmas are hybrid and co-genetic and, were formed from a similar mantle source, under a compressional regime prior to the opening of the present E–W-trending graben of the Aegean western Anatolian region.

Age and Regional Relationships of Granitoid Rocks of the Adirondack Highlands

Approximately 55% of the Adirondack highlands is underlain by granitic to tonalitic rocks that can be subdivided into five groups based on age, composition, and field occurrence: (1) early calcalkaline tonalites (ca. 1300 Ma) and associated granitic rocks (ca. 1250 Ma) of the southern and eastern highlands, (2) older anorogenic plutonic rocks (1145-1160 Ma) that occur throughout the highlands, (3) younger anorogenic plutonic rocks (ca. 1130 Ma) that envelop anorthosite massifs in the central highlands, (4) younger granitic rocks (1095-1105 Ma) of the northwest highlands, and (5) late leucogranitic rocks (1060-1080 Ma) that occur throughout the northern highlands. The data presented here are in good agreement with zircon ages from the Green Mountains of Vermont as well as from southern Ontario and Quebec and suggest similar geologic histories for these regions. These results document a protracted and complicated evolution for this portion of the Grenville Province.

K-Feldspar Content of Jurassic and Cretaceous Graywackes of Northern Coast Ranges and Sacramento Valley, California

Graywackes of Late Jurassic to Late Cretaceous age are the predominant rocks in the northern Coast Ranges and the western Sacramento Valley provinces in California. These graywackes are similar in appearance, but their content of K-feldspar, which can be readily estimated after selective staining, differs widely. Study of several hundred specimens indicates that K-feldspar content is helpful in correlating or distinguishing these rocks, and sheds light on the geologic history of western California. In the Sacramento Valley a conformable sequence of Knoxville (Upper Jurassic), Shasta (Lower Cretaceous), and Upper Cretaceous rocks progressively increases in average K-feldspar content with decreasing age. Graywackes of the Franciscan formation, lying in the Coast Ranges farther west, generally contain no K-feldspar, although some contain a trace, and a few of more uncertain affiliation contain large amounts. Another belt of graywackes, west of the Franciscan formation and paralleling the Pacific shore, contains nearly as much K-feldspar as the Upper Cretaceous rocks of the Sacramento Valley, and most of these rocks are probably of middle Cretaceous age. The writers attribute the increase of K-feldspar with decreasing age of the rocks of the Sacramento Valley to changes in the source area, which is believed to be the Klamath Mountains and the Sierra Nevada. The changes include an increase in proportion of granitic rocks to others as the areas were stripped, and more abundant K-feldspar in the younger granitic rocks. The high K-feldspar content in the middle Cretaceous rocks of the coastal belt suggests that the rocks were deposited in the same basin as the Sacramento Valley rocks.

GEOLOGY OF THE TEMISKAMING-GRENVILLE CONTACT SOUTHEAST OF LAKE TEMAGAMI, NORTHERN ONTARIO, CANADA

The area studied lies southeast of Lake Temagami in Northern Ontario and comprises 30 miles of the contact between the Temiskaming and Grenville sub-provinces, two divisions of the Canadian Shield. The age relationship between these sub-provinces has long been a problem, and the present work was undertaken to obtain further information toward a solution. The Grenville sub-province, in the southeast part of the area, is underlain by gneisses invaded by granite. In the area studied, the Temiskaming sub-province consists of granitic rocks overlain by sedimentary rocks of the Cobalt series which are cut by sills and dikes of diabase. The work has shown that the two sub-provinces are in fault contact along a zone of northeast-striking, east-dipping faults, with displacement of the Grenville, or southeast side, upward. The amount of displacement at these faults ranks them with the major tectonic features of the earth's crust. Movement has taken place many times, the latest in late Precambrian. This movement produced a lineation in all rocks of the area and especially in the Grenville sub-province. A second set of faults strikes northwest parallel both to the movement on the earlier northeast-striking faults and to a prominent set of vertical joints. Throughout the area, northeast-striking schistosity and foliation dip southeast, and the dip flattens progressively south-eastward. Metamorphism increases in grade in the same direction. In granitic rocks of the Temiskaming sub-province the biotite isograd and zones of increasing metamorphism parallel the contact between the sub-provinces. The gneisses in the Grenville sub-province immediately east of the contact are similar in chemical composition to the granites of the Temiskaming sub-province and are considered their metamorphosed equivalent. These gneisses are cut by younger granitic rocks that are high in potash that probably originated, in part at least, by granitization. Both these granites are pre-Cobalt and probably post-Temiskaming.