Showing papers on "Metamorphism published in 1969"

Le massif ultrabasique des Beni Bouchera (Rif Interne, Maroc): Etude des péridotites de haute température et de haute pression, et des pyroxénolites, à grenat ou sans grenat, qui leur sont associées

Abstract: The ultrabasic rocks which form the Beni Bouchera massif are schistose and folded isoclinally. They outcrop at the center of a late formed anticline under a series of granulite facies precambrian or lower paleozoic rocks. The ultrabasic rocks appear to have been in situ during the major structural development of the series and were emplaced at the time of metamorphism. The massif itself is essentially a peridotite, apparently layered, which contains pyroxenite bands (about 3% of the total volume of the massif). The peridotites are olivinerich containing two aluminous pyroxenes and spinel. The pyroxenites are however quite varied. They are frequently banded themselves and in several centimeters thickness can vary from enstatite rocks at the exterior of the layers to garnet clinopyroxenites at the center with websterite and garnet websterite as intermediate rock type. They are also present as monofacies bands of enstatite or garnet clinopyroxenite rock. Occasionally the minerals in these bands show exsolution features.

The Mesozoic of New Zealand: Chapters in the history of the Circum-Pacific Mobile Belt: 22ND WILLIAM SMITH LECTURE

Abstract: The Mesozoic includes the later history of the New Zealand Geosyncline, the Rangitata Orogeny that ended the geosynclinal phase, superposing a new structural system, and the beginning of the transgression that followed. The main elements of the geosyncline, from west to east, are: (1) a foreland, metamorphosed and intruded by granite in a relatively high temperature/low pressure environment; (2) a geosynclinal margin or hinge-line, the median tectonic line of Landis and Coombs; (3) a western or marginal belt of geosynclinal sediments (Hokonui Facies) relatively fossiliferous, apparently deposited over the Pacific edge of the sial and now forming a western marginal syncline; (4) an abrupt facies junction marked by ultramafic intrusions and Permian volcanics, perhaps the early Mesozoic oceanic boundary of sialic crust; and (5) an eastern belt of relatively unfossiliferous deformed sediments of the greywacke suite (Torlesse Facies), metamorphosed in a high pressure environment, their lower parts to schist. The western or Hokonui zone records rapid and almost continuous marine sedimentation during the Triassic and Jurassic, with some brief local interruption during the Middle Jurassic. The eastern or Torlesse zone of thick quartzo–feldspathic greywackes and argillites, with some spilitic lavas, occupying a Pacificmargin trench, has proved difficult to interpret owing to structural complexity, lithological monotony and scarcity of fossils. The metamorphic boundaries vary in age in different places, and the known fossils show a nonrandom distribution in age and space, as if basins of rapid sedimentation migrated up and down its axis throughout Permian to Tithonian time, reacting to variable thermal gradients. The Rangitata Orogeny began with precursor movements in the Middle Jurassic but climaxed in the Lower Cretaceous. The orogeny led to the main metamorphism and folding of the geosyncline, long the East phases of granite intrusion in the foreland, and the torsion and rupture of the geosyncline along the transcurrent Alpine Fault. In the Aptian, basins within the New Zealand geosyncline received sediment still of geosynclinal facies, so that it remains uncertain whether deposition was locally continuous or whether the Cretaceous is everywhere unconformable on underlying rocks. Synorogenic breccias (pre-Campanian) locally overlie schist and foreland granites that have been dated isotopically as mid-Cretaceous, and thus point to rapid uplift and erosion. By the Macstrichtian, freshwater and marine sediments indicate peneplanation and dominantly chemical weathering. The Tasman Sea is first evident in the Cretaceous, and the Cretaceous transgression, continuing in the Tertiary, reduced the land to a changing archipelago. Early Mesozoic structures are consistent with a simple continent–ocean boundary and trench trending north-west, parallel to the Mesozoic Darwin Rise. The Cretaceous orogeny superposed north-east trending structures that have dominated later history; their northern parts face the Pacific and parallel a section of the East Pacific Rise, but their southern parts face the Tasman Sea in a kind of scissors or ‘anceps’ structure. The change from a single Pacific margin to the present anceps structure facing both the Pacific and Tasman is consistent with post-Jurassic development of the Tasman and other marginal ocean basins during the Cretaceous orogeny, which entailed considerable horizontal movements of the crust. New Zealand geology can be extrapolated onto the surrounding shallow sea-floors, underlain by subcontinental crust, using evidence from outlying islands, submarine geology, and geophysics. The speculative process of rationalizing Lower Mesozoic palaeogeography is made easier if it is assumed that the early Mesozoic arc, now sinuous, was originally straighter, and that the marginal ocean basins now interrupting its course developed during the Cretaceous deformation that led to its curvature.

Contact Metamorphism in the Kwoiek Area of British Columbia: An End Member of the Metamorphic Process

Abstract: The Kwoiek area of British Columbia has undergone high-pressure contact metamorphism at the eastern flank of the Coast Range batholithic complex. The major prograde metamorphic reactions in the sedimentary rocks are: A possible metastable reaction occurring between equations 2 and 3 is Compositional zoning in individual garnets is principally the result of depletion of the constituent elements of the garnet from a homogeneous matrix as the garnet grows. Minerals such as garnet, which by growth can change the composition of the matrix due to the fact that they do not internally equilibrate in response to changing external conditions, are termed “refractory” phases. It is shown that staurolite and probably ilmenite are “refractory” phases. During resorbtion of the refractory phase ilmenite, iron is added to the matrix, which gives rise to decreasing Mg/Fe in the concurrently growing garnet and staurolite. Biotite, chlorite, and presumably muscovite are homogeneous minerals that continuously internally equilibrate in response to changing external conditions, including retrograde metamorphism; therefore, they are considered to form the matrix from which the refractory minerals grow. Differences between measured electron micro-probe profiles of garnet grains and those calculated for a depletion model are due mainly to the effect of Mn on the Mg/Fe ratio in the garnet and partly to changing relative proportions of chlorite, muscovite, and biotite during the growth of the garnet. Higher grade assemblages appear to be the result of bypassing of lower grade assemblages. This leads to a model of overstepping of reactions that is tested by an analysis of garnet zoning profiles from assemblages that contain a limiting garnet composition (that is, the garnet composition projects at a corner of an assemblage polyhedron). The composition of these garnets is a function of the temperature at which they grew, which was nearly constant for each garnet. Magnesian chlorite persists stably with sillimanite, in the absence of muscovite; staurolite with a moderate zinc content persists with quartz in the highest grade rocks of the Kwoiek area.

Oxygen and hydrogen isotope studies of contact metamorphism in the Santa Rosa Range, Nevada and other areas

Abstract: The O18/O16 and D/H ratios have been determined for rocks and coexisting minerals from several granitic plutons and their contact metamorphic aureoles in the Santa Rosa Range, Nevada, and the Eldora area, Colorado, with emphasis on pelitic rocks. A consistent order of O18/O16 and D/H enrichment in coexisting minerals, and a correlation between isotopic fractionations among coexisting mineral pairs are commonly observed, suggesting that mineral assemblages tend to approach isotopic equilibrium during contact metamorphism. In certain cases, a systematic decrease is observed in the oxygen isotopic fractionations of mineral pairs as one approaches the intrusive contacts. Isotopic temperatures generally show good agreement with heat flow considerations. Based on the experimentally determined quartz-muscovite O18/O16 fractionation calibration curve, temperatures are estimated to be 525 to 625° C at the contacts of the granitic stocks studied.

Reconnaissance Rb-Sr Dating of the Precambrian Rocks of Southern Peninsular India

Abstract: Many new Rb-Sr age determination analyses are reported for the crystalline complex of southern Peninsular India. These are mostly total-rock ages, often isochrons. Data are still insufficient for a reliable geochronology. Rocks of apparent ages ranging from over 3000 m.y. to 720 m.y. have been dated, and a lower Palaeozoic event reflected in mineral ages of about 500 m.y., known to have affected all Ceylon and much of the east coast of India, has been found as far north as Coimbatore. The oldest rocks have been found in Kerala, the Nilgiri Hills and southern Mysore. One age of 2700 m.y. has been found in Kerala. The craton of Mysore-Hyderabad has an age of at least 2585 ± 40 m.y., which is the apparent age of the Peninsular Gneiss over a wide area. The age of the Dharwar System remains uncertain, although the lavas near Chitradurga give an isochron at 2345 ± 60 m.y. The Chitradurga Granite has an age between 2450 and 2400 m.y. The Closepet Granite presents difficulties arising from its poor definition, but contains components between 2400 and 2000 m.y. Rocks of about 2100 m.y. occur in Kerala and western Tamizhagam, and could be present in Mysore, where an event at this time is recorded by biotite. The Chamundi Hill Granite of Mysore city, and a granite from the Ramanathapuram District of Tamizhagam give 790 ± 60 and 720 m.y. respectively, suggesting the possibility of widespread if sporadic intrusion in the southern half of the region at about this time. There is no real evidence yet of any major reflection of the Vijayan retrogressive metamorphism of Ceylon at 1140 m.y., except possibly in the intrusion of the Sivamalai soda-syenite. However, the known mineral ages of about 1690, 1650 and 1150 m.y. along the west coast suggest repeated marginal mobility of the Mysore-Hyderabad craton otherwise stable since about 2000 m.y. though intruded repeatedly by several dyke suites not yet dated. Much further sampling is needed, as well as the combination of several methods of dating. To facilitate this, modern geological and tectonic maps of medium scale are very desirable.

Structural and petrofabric analysis of an “Alpine-type” peridotite: the lherzolite of the French Pyrenees

Abstract: A narrow E-W-striking “graben” containing Mesozoic sediments is separated from the Hercynian mountain chain of the Pyrenees in the south by the North-Pyrenean fault and in the north by a detachment plane. In the region around Vicdessos and the Etang de Lers (Ariege, France), three Alpine phases of deformation are recognized: the first and strongest has an E-W-trending fold axis and a subvertical axial-plane cleavage, the second and weakest also has an E-W-trending axis but a northward dipping axial-plane cleavage, and the third a subvertical axis and subvertical N-S-striking axial-plane cleavage. The northernmost zone has ultramafic bodies consisting principally of lherzolite. This zone is also characterized by the occurrence of metamorphism and brecciation. The metamorphism is of a dynamo-thermal type, resembling the Abukumatype of metamorphism, with a climax during the first Alpine deformational phase. Petrofabric analyses of some metamorphic minerals (calcite, dolomite, phlogopite, scapolite, and tremolite) indicate that this was a flattening phase. Although the lherzolites did not suffer from the Alpine metamorphism, they show fracture cleavages of the first and the third Alpine phases. However, the lherzolites contain many more older structures, such as a layering of spinel pyroxenites, an isoclinal folding with an axial-plane cleavage, and a weak, more open folding of these structures. Petrofabric analyses of olivine, enstatite, and diopside indicate that the fabric of the lherzolites is determined by the axial-plane cleavage of the isoclinal folds (\u03b1-olivine, \u03b1-enstatite, and \u03b1-diopside lying perpendicular to the axial-plane cleavage, and \u03b3-olivine, \u03b3-enstatite, and \u03b3-diopside lying parallel to the fold axis) which arose from a pre-Alpine syntectonic recrystallization. In general, the olivine grains show a fabric habit, i.e. they are referable to a triaxial ellipsoid, the short axis perpendicular to the axial-plane cleavage, the long axis parallel to the fold axis, but these axes do not always coincide with the optic elasticity axes. All these phenomena form indications that the lherzolites intruded as solid blocks. There is no indication of a tectonic intrusion; faults in the country rock and shear zones and slickenside structures in the lherzolites are absent. The brecciation of the lherzolites, which occurred principally along their margins, and of the country rock just in the zone containing lherzolites, is definitely linked to the emplacement of the lherzolites. Indications that the breccias originated by explosions are provided by such features as the funnel-shaped breccia bodies and the discordant nature of the breccias, which cut across the bedding planes. This situation suggests that the solid intrusion of the lherzolites could have been caused by degassing of the upper mantle followed by gaseous explosions, just before the Alpine orogeny. It seems highly probable that the lherzolites originated in the upper mantle, where their pre-Alpine structures and fabric would also have been formed. The gabbroic intrusions (ophites) are truly magmatic, and are probably genetically related to the lherzolites, but this relationship would also date back to the upper mantle.

Burial Metamorphism of the Late Mesozoic Great Valley Sequence, Cache Creek, California

Abstract: In sandstones of the Great Valley Sequence of Mesozoic age at Cache Creek, post-depositional albitization of plagioclase and chloritization of biotite are widespread in noncalcareous rocks, but uncommon in rocks with calcareous cement. The degree of alteration increases systematically with age and inferred depth of burial in Upper Cretaceous strata, and is uniformly great in Lower Cretaceous strata that were buried from 20,000 to 30,000 feet. Laumontite is characteristic of thoroughly altered Lower Cretaceous rocks. Other metamorphic assemblages may be widespread at higher and lower horizons.

Remnants of an Early Metasedimentary Assemblage in the Lewisian Complex of the Outer Hebrides

Abstract: Remnants of a varied association of rocks, including psammitic, pelitic, semi-pelitic and calcareous metasediments, have been mapped in all the larger islands of the Outer Hebrides. Most of the remnants are concentrated in relatively narrow zones in which they are associated with, and pass into, quartzo-feldspathic gneisses. These zones are flanked by areas made predominantly of quartzo-feldspathic gneisses which appear to lack metasedimentary remnants. The metasediments were metamorphosed, migmatised and partially transformed into gneisses prior to the emplacement of the Scourie Dyke suite. Their least-modified portions are in many places intimately associated with bodies of basic rock which are thought to have shielded them from the effects of successive phases of metamorphism. Their present mineral assemblages and fabrics date from the Laxfordian episodes of regeneration. Relict mineral assemblages suggest that the earliest recognised Laxfordian metamorphism was of amphibolite facies over much of the Hebrides but was locally of granulite facies. The metasedimentary rocks are regarded as derivatives of a supracrustal group deposited prior to the main Scourian period of gneiss-formation and much of the gneiss is thought to be derived from such a group. A still older basement could, however, be represented by some of the areas of quartzo-feldspathic gneisses which intervene between the metasedimentary zones, though the existence of such a unit remains open to discussion.

Structure of continental margin off Sierra Leone, West Africa

Abstract: Seismic-refraction data from the ocean are combined with geological data from the shore to determine the structure of the continental margin and the continent-ocean transition off West Africa. More than 5 km of Cretaceous-Quaternary sediments accumulated on the continental shelf and slope as the surface of the underlying continental crust subsided. On the shelf the pre-Mesozoic rocks consist of a 2- to 3-km thickness of Cambrian-Devonian sedimentary rocks over a crystalline Precambrian continental crust. These Paleozoic sedimentary rocks can be interpreted as a pinching out under the upper continental slope off Sierra Leone and may not extend any great distance into the South Atlantic. Layer 2 (oceanic basement) with a velocity range similar to that of the Paleozoic sedimentary rocks appears to extend all the way from the mid-Atlantic ridge to the lower continental slope. This layer may consist entirely of post-Paleozoic volcanies. The Cretaceous-Quaternary embayment of the Senegal basin is separated from the less well developed embayment of Sierra Leone by a west-southwest-trending arch in the pre-Mesozoic surface under the continental shelf. Mesozoic or Tertiary faulting produced the present configuration of the arch, but this faulting probably followed the zones of Paleozoic folding and faulting along the northwest limb. An anomalously high-velocity crust, 7.0–7.3 km/sec, occurs near the Sierra Leone rise. This may be similar to the crust that occurs beneath the Bermuda rise, a crust formed by metamorphism due to the igneous activity of the many seamounts comprising the rise. This anomalous crust does not extend to the ridge flank, suggesting that the Sierra Leone rise is an independent center of volcanism.

Intrusive Sandstone Dykes in the Siamo Slate near Negaunee, Michigan

Abstract: Sandstone dykes up to 15cm wide and more than 3 m long transect the bedding and are parallel to the cleavage in the Siamo Slate of the Marquette synclinorium. These clastic dykes are common in slates of very low metamorphic grade east of Negaunee, and are composed of fine-grained sandstones commonly containing a higher proportion of iron-rich carbonate than the host rocks. The cleavage in both the slates and the interbedded sandstones is produced by parallel orientation of thin pelitic foliae. Individual cleavage foliae can be traced from one pelitic layer, through a sandy lamination and into the next pelitic layer. Microstructures in disrupted sandy laminations indicate that the sandstone dykes formed penecontemporaneously with the cleavage. Both the cleavage foliae and sandstone dykes are interpreted as having formed by intrusion accompanying tectonic dewatering of the Siamo Slate during late diagenesis or very low-grade metamorphism.

K-Rb ratio in high grade metamorphism: A confirmation of the hypothesis of a continual crustal evolution

Abstract: The K:Rb ratio (R) in a metamorphic sequence with increasing metamorphism of the West Alps has been analysed. By a regression analysis a comparison is made with the trends observed for R byShaw (1968) in many magmatic processes. The K:Rb ratio in the metamorphic rocks shows a marked increase with increasing metamorphism in relation to a removal of the K content. The arithmetic averages of R in the rocks of amphibolite and granulite facies are 231 and 505 respectively. In some samples of granulite facies the R values are higher than those of the continental alkalic basalts and approximate to the oceanic tholeiites. The variations in the K:Rb ratio may be imputed, according toShaw, to an anatexis process which took place under high grade metamorphic conditions. The marked increase of the K:Rb ratio in the deep material would be caused by partial separation between K and Rb in the granulite assemblages and by an upward migration of Rb, relative to K, from the deep crustal levels of granulite facies.

Age and correlation of the Nimrod Group and other precambrian rock units in the central Transantarctic Mountains, Antarctica

Abstract: Potassium-argon dates determined on hornblendes from amphibolites in the regionally metamorphosed gneisses and schists of the Nimrod Group at the head of the Nimrod Glacier range in age from 500 to 1,050 m.y. The younger dates agree with previously determined K—Ar ages on micas and probably indicate reheating of the metamorphic terrane during emplacement of the Granite Harbour Intrusives in the Ordovician. Precambrian ages clustered between 1,000 and 1,050 m.y. probably date the end of the regional metamorphism that is termed Nimrod Orogeny and is the earliest event known to have affected the Transantarctic Mountains. An event at about the same time has been recognised on the continental margin of east Antarctica between 90° E and 130° E and in western Queen Maud Land. Elsewhere it has been overprinted with loss of radiogenic argon and strontium by later events, especially the 450–520 m.y. Ross Orogeny. Late Precambrian orogenic activity, lasting between 620 and 680 m.y., may also b recorded in t...

Coal metamorphism and hydrocarbon potential in the Upper Paleozoic of the Atlantic Provinces, Canada

Abstract: Coal rank is used to measure the degree of organic metamorphism, which is of great importance in the evaluation of the hydrocarbon potential of a region.The rank-is determined on true coal seams an...

Zircon U-Pb and Whole-Rock Rb-Sr Ages and Early Crustal Development near Rainy Lake, Ontario

Abstract: Early Precambrian rocks near Rainy Lake, on the Ontario-Minnesota border, are comprised of a thick section of metasediments and paragneisses (Coutchiching) overlain conformably by metavolcanics (Keewatin) and intruded by Laurentian Igneous rocks. Eight of ten zircon samples from these units lie within experimental error (±30 m.y.) of a 2750-m.y. Concordia chord. We conclude that the zircon ages are primary and not a result of metamorphic resetting. There is thus no evidence for an older sialic source region from which the detrital Coutchiching zircons were derived. It is probable that these detrital zircons were derived locally from contemporaneous volcanics and shallow intrusives within the geosyncline. There is little recorded time difference ( The Rb-Sr isochron for the volcanics gives an age of 2770 m.y. (initial Sr 87 /Sr 86 = 0.7010) which is not statistically different from the Coutchiching isochron age of 2690 m.y. (initial ratio = 0.7002). Both isochrons are in agreement with the zircon intercept age of 2750 m.y. (using λRb 87 = 1.39 X 10 −11 yr −1 ). At least some of the paragneisses are shown to be open to strontium migration during regional metamorphism. The Coutchiching isochron age of 2690 m.y. probably represents an early stage of metamorphism during the Algoman orogeny. Samples of one Laurentian intrusive give an isochron age of 2520 m.y., which is distinctly younger than the ages of the Laurentian zircons. This intrusive has been highly sheared and the rocks have probably been reset during the shearing event.

The origin of the Connemara migmatites of the Cashel district, Connemara, Ireland

Abstract: After the Connemara schists, of Dalradian age, had been folded (F1) and metamorphosed to sillimanite grade, they were strongly hornfelsed by the intrusion of basic magma. From this magma crystallized peridotites, norites, pyroxenites and gabbros, all with An80–87 plagioclase. Movements then caused the magma to break up these ultrabasic rocks and crystallization of basic rocks occurred. They are labradorite (An55–65)-hornblende-relic clinopyroxene rocks that grade into quartz-hornblende-labradorite (An55)-gneiss, quartz-hornblende-labradorite (An55)- and andesine (An40)-gneiss and quartz-andesine (An40)-hornblende-biotite-gneiss. These gneisses formed during strong F2 and F3 movements, and probably during amphibolite-facies metamorphism, and they broke up, injected, metasomatized and amphibolized the earlier basic and ultrabasic rocks, giving a heterogeneous migmatite that commonly contains two plagioclases, An40 and An55. Quartzo-feldspath-ization of metasediment occurred, possibly up to a mile from the migmatites. This, and recrystallization, formed quartz-andesine (An40)-biotite-gneiss that merges into the magmatically crystallized gneisses as do gneisses formed by silicification of basic and ultrabasic rocks. Last to crystallize was K-feldspar gneiss, possibly formed by segregation of residual magma into one zone. This migmatite complex contains evidence of both magmatic and metamorphic crystallization of hornblende and the relative importance of each process remains to be determined. Tight F3folding formed the Cashel syncline, and a series of complementary folds. Later retrogressive metamorphism caused widespread chloritization, sericitization and saussuritization. Last of all was the intrusion of the Galway and Roundstone granites followed by their associated dykes. Twenty-four chemical analyses and nearly five hundred plagioclase determinations are utilized.

Age and Origin of the Oliverian Domes, Central-Western New Hampshire

Abstract: New information on the origin of mantled gneiss domes comes from a study of the Oliverian Domes in western New Hampshire. Fieldwork indicates that the core of the Mascoma Dome (a representative Oliverian Dome) can be subdivided into two major units: (1) massive gneiss (stratified core-rock) of intermediate igneous composition lying stratigraphically beneath the Ordovician Ammonoosuc Volcanics, and (2) a sub-central pluton of granite and quartz monzonite (unstratified core-rock) which crosscuts the massive gneiss and probably the Ammonoosuc Volcanics, but which lies unconformably beneath the Late Lower Silurian Clough Formation. Within limits imposed by analytical uncertainty and the metamorphic disturbance of the rocks, an Rb-Sr age of 440 ± 40 million years (initial Sr 87 /Sr 86 = 0.706 ± 0.002) is determined for whole-rock samples of the granitic sub-core of the Mascoma Dome, and for whole-rock samples of the Ammonoosuc Volcanics. Zircon separates from both the gneissic and granitic units within the core of the Mascoma Dome yield Pb 207 /Pb 206 ages of 450 ± 25 million years. The data indicate that the dome formed in the following stages: (1) Ordovician volcanism followed by intrusion of granitic rocks, (2) uplift and local unroofing followed “by deposition of Lower Silurian through Lower Devonian strata, and (3) garnet- to staurolite-grade post-Lower Devonian metamorphism and deformation. The crosscutting relationships were established by Ordovician plutonic activity and not by post-Lower Devonian plutonic activity or anatexis. The core-rock of the dome appears to be the result of volcanic and intrusive activity towards the end of the Ordovician, and not the result of in-place remobilization or anatexis of Precambrian basement subsequent to deposition of the mantling strata. The other Oliverian Domes, particularly those in New Hampshire, resemble the Mascoma Dome, and probably originated in much the same manner.

Petrography and Ages of Crystalline Basement Rocks of Florida--Some Extrapolations

Abstract: The crystalline rocks of the Florida basement are predominantly ignimbrite, tuff, and agglomerate, generally of rhyolitic composition but including individual clasts and, in places, whole sections (e.g., Humble No. 1 Campbell and No. 1 Jameson) of intermediate or basic composition. Kaolinite in some ignimbrite may be a relic of Early Cretaceous weathering. Incomplete adjustment to greenschist-facies conditions is seen, especially in the less acidic rocks, but may be of local extent. Regional metamorphism is not recognized. Alteration to quartz and 1M white mica is present regionally but probably represents diagenetic alteration of primary glass in the presence of alkaline formation water rather than metamorphism in the strict sense. Known intrusive rocks in Florida and Georgia are distributed sporadically except in an area of central Florida where a granitic province can be outlined. Granitic rocks are generally altered; specifically, in the Humble No. 1 Carroll the quartz monzonite is cataclastically shattered and veined but not pervasively sheared. The quartz-bearing hornblende diorite of the Sun No. 1 Powell Land Co. is a sill below which is hornfels derived from clayey volcanic-quartzose sandstone. The sandstone is the southernmost sedimentary rock known in the Florida basement. The Amerada No. 2 Cowles Magazines Inc. penetrated diabase or trachydiabase overlying, and in apparent fault contact with, regional metamorphic rocks. These rocks are mainly quartz-bearing hornblende-andesine amphibolite containing layers of glassy-looking leucocratic quartz diorite gneiss that superficially resembles vein quartz. Retrograde metamorphism under conditions of the zeolite facies converted the amphibolite-facies rocks to quartz-albite-chlorite-prehnite schist along shear zones; in the surviving amphibolite and quartz diorite gneiss, it converted biotite to chlorite plus prehnite and produced dusty alteration products in andesine. During the waning stages of the retrograde metamorphism, tension cracks in amphibolite and quartz diorite gneiss were filled with laumontite, mo oclinic potassium feldspar, calcite, and chlorite(?). The two southernmost basement wells penetrated basalt, perhaps presaging a distinct province of mafic extrusive rocks in southern or southwestern Florida, coincident with northwest-trending End_Page 283------------------------ magnetic and gravity highs in peninsular Florida and a belt of irregular magnetic anomalies over part of the West Florida shelf. In the Humble No. 1 Keen the basalt is unmetamorphosed and unweathered. It resembles submarine basalt and may be related to buried seamounts, which are suggested by the magnetic anomalies over the West Florida shelf. The basalt may be much younger than the acidic volcanic rocks. The age of the rhyolitic ignimbrite is unknown. Arguments can be presented for late Paleozoic, Triassic, or Jurassic ages, and also for a Precambrian age. The ignimbrite may belong to an extensive acidic igneous terrane that supplied considerable detritus to Cretaceous sedimentary rocks penetrated in the Cowles and Powell wells, and undoubtedly others. The amphibolite of the Cowles well is 530 m.y. old, or older. It is probably in a branch of the Damaran (or Pan-African) orogen. The apparent event, about 530 m.y. ago, involved the amphibolite-facies and retrograde zeolite-facies metamorphism of the Cowles amphibolite, emplacement or metamorphism of the Carroll quartz monzonite, and, questionably, emplacement of the Powell diorite sill which is at least 480 m.y. old. Contact metamorphism by the sill may have affected already metamorphosed Precambrian rock or sediment derived at least in part from Precambrian source rock. The isotopic data on these points are conflicting. Meager data suggest that the diabase in the Cowles well may be of late Paleozoic, Triassic, or Jurassic age.