Intrusive Rocks
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2021 ◽  
Author(s):  
Brenton Tozer

<p>The study of intrusive igneous rocks can provide insights into deep crustal processes. In active intraoceanic arc environments, the opportunity to study of these intrusive igneous rocks usually comes from xenoliths entrained within eruptive products, as accessibility to in situ intrusive rocks is limited. This thesis documents a suite of the first intrusive samples dredged from the Havre Trough, which provide insights into deep magmatic processes in this intraoceanic back-arc system. The suite of ten intrusive igneous rocks were dredged from Basin E, a back-arc basin (BAB) in the Kermadec Arc-Havre Trough (KAHT) and consist of in situ gabbroic meso- to orthocumulates. Four representative samples were selected from the suite of ten on the basis of grainsize, and from them a petrogenetic model was built to determine the associations of the samples within the magmatic system of the region. The four samples all exhibit comparable mineral assemblages of plagioclase, clinopyroxene, and magnetite, with olivine and orthopyroxene absent. Texturally the samples appear to have formed in (a) magma chamber(s) where the minerals cooled slowly and formed relatively large, euhedral crystals that trapped interstitial melt between them. The interstitial melts crystallised forming more evolved intercumulus material (plagioclase + quartz ± amphibole ± apatite). Three of the four samples have coarser grainsizes (1-2 mm), and exhibit similar magnetite temperature estimates, indicating that they formed from similar melts. The other sample has a finer grainsize (<1 mm), and exhibits lower temperature estimates, indicating that this sample formed from a lower temperature, faster cooling melt. Plagioclase compositions follow a similar trend to plagioclase phenocrysts from modern back-arc volcanoes which indicates that these samples have an association with the modern magmatic system rather than the now extinct Miocene (Colville) Arc. Clinopyroxene trace element data are also consistent with these samples being associated with the modern subduction system. The magma chamber(s) that the samples formed in, comes from a mid-lower crustal depth, 3-6 km based on pressure estimates from amphibole crystal chemistry. The exposure of rocks from this depth would have been facilitated by normal faulting associated with rifting and opening of the Havre Trough. Petrologic and geochemical analyses of these cumulates suggest that the deep, back-arc basins consist of entirely new magmatic material formed from BAB volcanism, with no evidence for pre-existing crust.</p>


2021 ◽  
Author(s):  
Brenton Tozer

<p>The study of intrusive igneous rocks can provide insights into deep crustal processes. In active intraoceanic arc environments, the opportunity to study of these intrusive igneous rocks usually comes from xenoliths entrained within eruptive products, as accessibility to in situ intrusive rocks is limited. This thesis documents a suite of the first intrusive samples dredged from the Havre Trough, which provide insights into deep magmatic processes in this intraoceanic back-arc system. The suite of ten intrusive igneous rocks were dredged from Basin E, a back-arc basin (BAB) in the Kermadec Arc-Havre Trough (KAHT) and consist of in situ gabbroic meso- to orthocumulates. Four representative samples were selected from the suite of ten on the basis of grainsize, and from them a petrogenetic model was built to determine the associations of the samples within the magmatic system of the region. The four samples all exhibit comparable mineral assemblages of plagioclase, clinopyroxene, and magnetite, with olivine and orthopyroxene absent. Texturally the samples appear to have formed in (a) magma chamber(s) where the minerals cooled slowly and formed relatively large, euhedral crystals that trapped interstitial melt between them. The interstitial melts crystallised forming more evolved intercumulus material (plagioclase + quartz ± amphibole ± apatite). Three of the four samples have coarser grainsizes (1-2 mm), and exhibit similar magnetite temperature estimates, indicating that they formed from similar melts. The other sample has a finer grainsize (<1 mm), and exhibits lower temperature estimates, indicating that this sample formed from a lower temperature, faster cooling melt. Plagioclase compositions follow a similar trend to plagioclase phenocrysts from modern back-arc volcanoes which indicates that these samples have an association with the modern magmatic system rather than the now extinct Miocene (Colville) Arc. Clinopyroxene trace element data are also consistent with these samples being associated with the modern subduction system. The magma chamber(s) that the samples formed in, comes from a mid-lower crustal depth, 3-6 km based on pressure estimates from amphibole crystal chemistry. The exposure of rocks from this depth would have been facilitated by normal faulting associated with rifting and opening of the Havre Trough. Petrologic and geochemical analyses of these cumulates suggest that the deep, back-arc basins consist of entirely new magmatic material formed from BAB volcanism, with no evidence for pre-existing crust.</p>


Geosphere ◽  
2021 ◽  
Author(s):  
Gregory J. Walsh ◽  
John N. Aleinikoff ◽  
Robert A. Ayuso ◽  
Robert P. Wintsch

Crustal fragments underlain by high-grade rocks represent a challenge to plate reconstructions, and integrated mapping, geochronology, and geochemistry enable the unravelling of the temporal and spatial history of exotic crustal blocks. The Quinebaug-Marlboro belt (QMB) is an enigmatic fragment on the trailing edge of the peri-Gondwanan Ganderian margin of southeastern New England. SHRIMP U-Pb geochronology and geochemistry indicate the presence of Ediacaran to Cambrian metamorphosed volcanic and intrusive rocks dated for the first time between ca. 540–500 Ma. The entire belt may preserve a cryptic, internal stratigraphy that is truncated by subsequent faulting. Detrital zircons from metapelite in the overlying Nashoba and Tatnic Hill Formations indicate deposition between ca. 485–435 Ma, with provenance from the underlying QMB or Ganderian crust. The Preston Gabbro (418 ± 3 Ma) provides a minimum age for the QMB. Mafic rocks are tholeiitic with trace elements that resemble arc and E-MORB sources, and samples with negative Nb-Ta anomalies are similar to arc-like rocks, but others show no negative Nb-Ta anomaly and are similar to rocks from E-MORB to OIB or backarc settings. Geochemistry points to a mixture of sources that include both mantle and continental crust. Metamorphic zircon, monazite, and titanite ages range from 400 to 305 Ma and intrusion of granitoids and migmatization occurred between 410 and 325 Ma. Age and chemistry support correlations with the Ellsworth terrane in Maine and the Penobscot arc and backarc system in Maritime Canada. The arc-rifting zone where the Mariana arc and the Mariana backarc basin converge is a possible modern analog.


Minerals ◽  
2021 ◽  
Vol 11 (6) ◽  
pp. 619
Author(s):  
Namhoon Kim ◽  
Sang-Mo Koh ◽  
Byoung-Woon You ◽  
Bum-Han Lee

The axinite-bearing Gukjeon Pb–Zn deposit is hosted by the limestone, a member of the Jeonggaksan Formation, which, in turn, forms the part of the Jusasan subgroup of the Yucheon Group in the Gyeongsang Basin in the southeastern part of the Korean Peninsula. In this study, we attempted to interpret the spatial and temporal relationships among geologic events, including the mineralization of this deposit. We constructed a new 3D orebody model and suggested a relationship between skarn alteration and related mineralization. Mineralization timing was constrained using SHRIMP zircon age dating results combined with boron geochemistry on coeval intrusive rocks. Skarn alterations are restrictively found in several horizons of the limestone formation. The major skarn minerals are garnet (grossular), pyroxene (hedenbergite), amphibole (actinolite and ferro-actinolite), axinite (tizenite and ferro-axinite), and epidote (clinozoisite and epidote). The three stages of pre-skarn, syn-skarn, and post-skarn alteration are recognized within the deposit. The syn-skarn alteration is characterized by prograde metasomatic pyroxene and garnet, and the retrograde metasomatic amphibole, axinite, and epidote. Major skarn sulfide minerals are sphalerite, chalcopyrite, galena, and pyrite, which were predominantly precipitated during the retrograde stage and formed amphibole and axinite skarns. The skarn orebodies seem to be disc- or flat-shaped with a convex form at the central part of the orebodies. The vertical ascending and horizontal infiltration of boron-rich hydrothermal fluid probably controlled the geometry of the orebodies. Considering the whole-rock major, trace, and boron geochemical and geochronological results, the timing of Pb–Zn mineralization can be tightly constrained between the emplacement of boron-poor intrusion (fine-grained granodiorite, 82.8 Ma) and boron-rich intrusion (porphyritic andesite in Beomdori andesitic rocks, 83.8 Ma) in a back-arc basin setting. The boron for mineralization was sourced from late Cretaceous (Campanian), subduction-related magmatic rocks along the margin of the Pacific plate.


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