Geochronology and geochemistry of the Early Cretaceous Jigongshan and Qijianfeng batholiths in the Tongbai orogen, central China: implications for lower crustal delamination
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Batholith
Hornblende
Porphyritic
Diorite
K-Ar mineral ages have been determined for late Cretaceous to early Tertiary granitic plutons in Ulsan-Kyeongju area, the mid-eastern Kyeongsang Basin. The granitic plutons in the area can be grouped into two plutonic units; the Ulsan and Kyeongju plutons. The Ulsan plutons consist of hornblende biotite granodiorite, Ulsan granitic complex and porphyritic biotite granite, and the Kyeongju plutons are composed of hornblende biotite granodiorite, biotite granite and alkali granite. K-Ar biotite ages (67∼51 Ma) of the Ulsan plutons are significantly older than those (50∼47 Ma) of the Kyeongju plutons. Combined with the previous results about the cooling history of adjecent plutons and petrological information of the studied plutons, Ulsan hornblende biotite granodiorite and Ulsan granitic complex, which have K-Ar biotite ages older than 62 Ma, are thought to have been emplaced during late Cretaceous. On the other hand, Ulsan porphyritic biotite granite, Kyeongju biotite granite and Kyeongju alkali granite, which show K-Ar biotite ages younger than 54 Ma, seem to have been formed during early Tertiary. However, it is impossible to infer the emplacement age of Kyeongju hornblende biotite granodiorite, because its K-Ar biotite ages are considered to have been reset by later thermal effect. Considering the existence of early Tertiary plutons in the study area, particularly Kyeongju area, is likely to be the youngest granitic province or segment in the Kyeongsang Basin.
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We report adventages of employing MgO as a differentiation index for the Namwon granitic complex. It is shown to be much more sensitive than the usual Harker index. The complex can be divided into two groups on the basis of /MgO ratio. The low /MgO group consists of hornblende biotite tonalite-granodiorite, porphyritic hornblende biotite granodiorite (PHBGd) and part of biotite granite (loBG). PHBGd shows its own distinct variation in the low group. This group is characterized in most cases by the presence of hornblende, even if it occurs as a trace amount. The high /MgO group consists of part of biotite granite (hiBG) and two mica granite. The major element differences between rock types are also apparent in biotite chemistry. These chemical data indicate that at least two distinct origins of magma are rquired for the complex. Two kinds of biotite granite revealed in this study show distinct geographic distribution, suggesting that a new geologic map should be made.
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At various places, such as the north side of Garabal Hill, the two Garabal burns (loc. iv and v, Fig. 1), and elsewhere, remarkably coarse rocks are found. A good section showing an apparent passage from tonalite to a coarse hornblendite is exposed at loc. v (Fig. 1). The rock in the bed of the burn is the normal tonalite, which appears to pass gradually to diorite. A closer examination of the unweathered rocks, however, shows that the passage is only apparent, and that the tonalite is clearly intrusive into the diorite with sharp junctions. The diorite near the junction is a rock of porphyritic aspect, containing large crystals of zoned diopside, abundant green hornblende and felspar, while the amount of quartz is small. A good deal of biotite is present, so that the rock might be described as a pyroxenemica-diorite. Within a few feet the rock becomes coarser in texture, and the felspathic content diminishes, while there is a corresponding increase in the amount of amphibole. The latter is the green hornblende common to the diorites, and is occasionally replaced by mosaics of secondary actinolite.
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K-Ar and Sr isotopic age determinations were made for Mesozoic granitic rocks intruding into Precambrian gneisses and schists in the Inje-Hongcheon district, South Korea. They include hornblende-biotite granodiorite (K-Ar biotite ages of 154 to 165 Ma), porphyritic biotite granite (149 to 157 Ma) and two-mica granite (149 to 155 Ma). The hornblende-biotite granodiorite gives K-Ar hornblende ages of 168 to 180 Ma and a Rb-Sr whole-rock isochron age of 212.0±26.6 Ma with an initial 87Sr/86Sr ratio of 0.71198±0.00046. These granitic rocks have been derived from crustal melt during late Triassic to early Jurassic time. The cooling rate of the hornblende-biotite granodiorite was slow in early stage and became greater as it approached shallow depths.
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The Precambrian Cachoeirinha-Salgueiro Fold Belt (CSF) located in the western portion of the states of Pernambuco and Paraiba is intruded, in its northern portion, by several coarsely porphyritic potassic calc-alkalic .batholiths. These batholiths were syntectonicaly unplaced in relation to the Brasiliano cycle (=Pan-African) and are commonly associated with potassium diorites suggesting coexistence and mixing between felsic and mafic magmas. In the Itaporanga batholith three petrographic domains-were mapped. A hybrid zone characterized by intense mechanical mixing of granite to granodiorite and potassium diorite magmas is located towards the border of the batholith. A commingling zone where felsic porphyritic granite to granodiorite and potassium diorite rocks are individualized at outcrop scale is located towards the center of the batholith. Finally a felsic porphyritic fades occur in the hybrid zone. The intense mechanical mixing observed in the hybrid zone developed migmatite-like structures (stromatic and less frequently agmatic), pillov -like structures, mafic enclaves with irregular shape and cuspate contacts between mafic and felsic rocks, suggesting diffusion of chemical species across contact. Major and trace element plotted against SiO2 agree with a mixing model to explain the hybrid samples plotted at intermediate position between felsic and mafic rocks. Similarity among chemical analyses of amphiboles from potassium dioritic enclaves of the Itaporanga batholith and from the potassium diorite stock east of it suggest a common source for both magmas. This hypothesis is corroborated by similar REE patterns for potassium dioritic enclaves of the Itaporanga batholith and for the potassium diorite stock. The batholiths shows a well developed foliation which dips towards its core suggesting that the present level of exposure represents the root zone of a diapir, where intense interaction between felsic and mafic magmas took place.
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The granitic masses of the western half of the Waswanipi map-area in northwestern Quebec are of two general types-(1) reddish to gray, coarse-grained, somewhat porphyritic, gneissic quartz-diorites, and (2) red to gray, medium- to fine-grained, quartz-diorites to granodiorites, and rarely soda granites. The rocks surrounding the batholiths are Keewatin-type volcanics, sediments, and anorthosite. Field evidence indicates that the gneiss is older than the massive quartz-diorite, and mineralogic and textural evidence indicates that the gneissic type represents a metamorphosed shell of invaded rocks altered by emanations from a solidifying batholith. The medium-to fine-grained type represents an intermediate zone between metamorphosed country rock-forming Type I and still later solidifying magma.
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