Country rock

About: Country rock is a research topic. Over the lifetime, 875 publications have been published within this topic receiving 18897 citations.

A neodymium and strontium isotopic study of the Mesozoic calc‐alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California

Abstract: Plutonic igneous rocks of the Sierra Nevada batholith exhibit a range of Nd isotopic composition described by eNd = +6.5 to −7.6. Similar rock types from the Peninsular Ranges have eNd = +8.0 to −6.4. In both batholiths, eNd correlates strongly with initial 87Sr/86Sr. Decreasing eNd values are accompanied by increasing 87Sr/86Sr and increasing δ18O; the correlation with δ18O being more pronounced for the Peninsular Ranges. The eNd values show regular geographic variations, as was found previously for initial 87Sr/86Sr. Three metasedimentary country rock samples from the Sierra Nevada region have low eNd values (−11 to −16) and Precambrian model Sm-Nd ages (1.5 to 1.9 AE). The country rock eNd values, and those of primitive oceanic island arcs (eNd = +8), bracket the data for the batholith rocks. The Nd, Sr, and O isotopic data can be explained if the batholiths are mixtures of island arc and metasedimentary components, the latter being of both Paleozoic and early Proterozoic age. This model appears to be consistent with existing Pb isotopic data. Consideration of O-Sr isotopic relations and the variation of 147Sm/144Nd with eNd suggests that assimilation of crustal rocks by magmas rising from the mantle and undergoing fractional crystallization could have been the major process responsible for the mixing of crustal- and mantle-derived components. The isotopic data, when combined with assumptions about the structure of the crust beneath the batholiths, suggest that about 50% of the crustal material presently within the geographic boundaries of the batholiths and above the Moho represents juvenile crust derived from the mantle in the Mesozoic. The remaining material appears to be mostly derived from 1.8-AE crust, yielding an average crust formation age of nearly 1 AE for this section of the crust. This result, which may apply to large portions of the Cordillera, suggests that the average age of the North American continent may be greater than previously estimated. The concentration of Nd correlates well with eNd in the batholith rocks and supports the conclusion that juvenile continental crust is derived from mantle reservoirs that are depleted in incompatible elements. A 1.5-AE Sm-Nd model age for sedimentary rocks of the Mesozoic(?) Calaveras Formation indicates that the Nd in this “oceanic” terrain is dominated by continental detritus and demonstrates the potential of Sm-Nd isotopic studies for aiding in construction of tectonic models.

Serpentinization: a review

Abstract: The common alteration assemblage produced by serpentinization of ultramafic rocks is: lizardite, chrysotile, magnetite±brucite±antigorite. Lizardite-chrysotile serpentinites are more common than antigorite; the presence of antigorite indicates that the serpentinite has undergone prograde metamorphism or that the periootite was serpentinized in a higher P,T regime than lizardite and chrysotile. The iron subsitution into serpentine minerals and brucite is a function of temperature at low f O 2 , with increased temperature enhancing magnetite formation. The presence of awaruite and native Fe are strong evidence for a locally very reducing environment. Isotopic studies have shown a wide variety of origins for the fluids involved in serpentinization. The increased boron content of serpentinized rocks when compared to boron contents of the parent ultramafic body indicates a possible sea water origin for the fluids. Serpentinization takes place under both constant volume and constant chemical composition conditions. The factors in evaluating the importance of the two processes for an individual serpentinite are: (1) determination of the mineral assemblage and its paragenesis, (2) the structural and tectonic relationship of the ultramafic body to its country rock, (3) fluid access to the rock in duration and amount, and (4) timing of serpentinization - before, during or after emplacement into the crust.

Depositional features and stratigraphic sections in granitic plutons: implications for the emplacement and crystallization of granitic magma

Abstract: Many granitic plutons contain sheet-like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature, more mafic magma during crystallization of the granitic plutons. Small-scale structures associated with these bodies (e.g. load-cast and compaction features, silicic pipes extending from granitic layers into adjacent gabbroic sheets) indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber (on granitic crystal mush and beneath crystal-poor magma) while the mafic magma was incompletely crystallized. These structures indicate ‘way up', typically toward the interior of the intrusions, and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition. Hence, these plutons preserve a stratigraphic history of events involved in the construction (filling, replenishment) and crystallization of the magma chamber. The distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions, which usually preserve mutual intrusive contacts and ‘dike–sill’ relations of different magma types. The considerable thickness of material that can be interpreted as depositional, and the evidence for replenishment, suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton. Thus, magma chambers may be constructed much more slowly than presently envisaged. The present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber (along with the underlying solidified granite and country rock) sank during continuing episodes of magma chamber replenishment. These internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber.

Sudbury Structure as an Astrobleme

Abstract: The Sudbury structure is interpreted as a 17-billion-year-old asteroid impact structure or "astrobleme" created by a $$3 X 10^{29}$$ erg event If traveling at 15 km/sec, the asteroid was about 4 km in diameter A crater was formed 30 miles across and 2 miles deep Along with melted country rock, the bolide, possibly a copper-rich nickel-iron meteorite, is still partially preserved, although converted to sulfides, as a marginal sheet along the crater wall and as injections into radial tension cracks Shock brecciation and rock were heaved up, forming the crater wall The collar is still easily recognized along the southern periphery of the Sudbury structure, but it can only be permissibly assumed in the massive granitic rocks around the northern periphery Because of its great magnitude, the Sudbury event triggered magmatism by offloading the lower crust and mantle and by adding shock heat Partial fusion of already critically warm rock resulted A saucer-shaped pool of magma, an extrusive lopolith, for

Ultrahigh-pressure metamorphic rocks in the Dabieshan-Sulu region of China

Abstract: 1. A Review on Researches of UHP Rocks in the Dabieshan-Sulu Region Cong Bolin, Wang Qingchen. 2. Regional Geology Wang Qingchen, et al. 3. Structural Geology Wang Qingchen, et al. 4. Petrography of UHPM Rocks and Their Country Rock Gneiss Zhang Ruyuan, et al. 5. Major and Trace Element Geochemistry of Eclogites and Related Rocks Zhai Mingguo, Cong Bolin. 6. Isotopic Geochronology Li Shuguang. 7. Mineralogy of UHP Rocks Zhang Ruyuan, et al. 8. Metamorphic Evolution of UHPM Rocks Cong Bolin, et al. 9. Tectonic Evolution of UHP Rocks Wang Qingchen, et al. Appendix. Reference. Epilogue. Index.