The Weekend dykes consist of 10 Late Devonian spessartite lamprophyres cropping out within the allochthonous Meguma lithotectonic terrane of the northern Appalachians. The dykes have characteristic panidiomorphic textures, with seriate phenocrysts of amphibole, clinopyroxene, and rare biotite set in a groundmass of intergrown plagioclase, K-feldspar, and quartz, with deuteric calcite and epidote. All dykes intruded during one magmatic episode (ca. 370 Ma) following terrane accretion of the Acadian Orogeny. The unaltered Weekend dykes show restricted major element variation (SiO 2 54–58 wt.%, Al 2 O 3 14–16 wt.%, MgO 7–11 wt.%, and total alkalies 2.4–5.5 wt.%) and have high Mg# (71–80) and moderate to high concentrations of Ni (69–278 ppm) and Cr (390–992 ppm). Large ion lithophile element (e.g., Sr, Ba 294–1194 ppm) and light rare earth element (13–67 CN ) abundances are high relative to high field strength element (e.g., Nb, Ta, Y 0.45–26 ppm) and heavy rare earth element (6–30 CN ) abundances. Geochemical variation largely corresponds to minor phenocryst fractionation, but high Mg# indicate the primitive nature of most dykes and preclude significant evolution of lamprophyric magmas in the crust. Incompatible element enrichments coupled with depleted mantle high field strength element abundances probably require a melt derived from reenriched lithospheric mantle sources, whereas Nb depletion and the volatile-rich mineralogy suggest metasomatic contributions from subducted ocean lithosphere. Geochemical comparisons with continental margin arc basalts and immobile element tectono-magmatic discrimination reinforce a subduction model for the Weekend dykes and strongly suggest active subduction prior to the emplacement of the Meguma terrane.
The Early Cretaceous (∼115 Ma) Zarza Intrusive Complex is a small (<10 km2), bimodal ring complex that may represent a magmatic microcosm of the western Peninsular Ranges batholith. Its tholeiitic gabbro bosses (25% by area; Al2O3 > 17 wt %, Sr < 463 ppm) formed at subvolcanic depths <0.2 GPa (8 km) by >30% plagioclase accumulation from andesitic magma batches now preserved as cone-sheets (63%; SiO2 ≥ 55%, MgO < 3.1%, Ni ∼30 ppm). Quenched cone-sheets are polymorphic (olivine–pyroxene- or hornblende-bearing) and share similar chemical and isotopic compositions (εNd +7, 87Sr/86Srt = 115 < 0.704) that preclude extensive sediment contamination. Their calc-alkaline basalt parents apparently contained very different volatile concentrations (∼3–7 wt % H2O) inherited from various equilibria between subduction-related aqueous fluids, and depleted lherzolite in the upper mantle. Recharge and/or dominant ferromagnesian mineral fractionation at ∼0.8 GPa (>28 km) depth best explains subsequent differentiation towards high-Al andesite. Contemporaneous tonalite (SiO2 64–74%, molar Al2O3/(CaO + Na2O + K2O) [A/CNK] > 1.0, 87Sr/86Sri 0.703) probably formed in situ by andesite fractionation, whereas spatially associated trondhjemite (A/CNK > 0.98, 87Sr/86Sri 0.702) is more consistent with 8–19% dehydration melting of metabasite in the contact aureole. Enrichments of incompatible K2O, Ba, Rb and Th in all silica-saturated rocks from the western part of the batholith can be explained by mixing between different proportions of fractionated and partially melted end-members generated within thick oceanic arc basement.
Four mid- to late Devonian peraluminous granitoid intrusions in the Meguma Zone of southwestern Nova Scotia contain abundant enclaves typical of orogenic granitoid bodies. The Barrington Passage and Shelbume plutons contain an assemblage of granoblastic metasedimentary homfelsic enclaves (49%) that have aluminosilicate porphyroblasts, and surmicaccous enclaves (51%) that consist of > 70% decussate biotitc with apatite and zircon inclusions. Metasedimentary enclaves pre dominate in the Port Mouton Pluton and the South Mountain Batholith (52%), but these intrusions also contain abundant microgranular and coarse-grained granitoid enclaves (25% and 23%, respectively) that have peraluminous mineral assemblages and tonalitic to leucomonzogranitic compositions. High concentrations of metasedimentary enclaves at the country rock contacts suggest that they probably formed as xenotiths stoped from the Meguma Group. No xenoliths reflect palaeosomes of basement gneiss from the protolith of the granitoid melts, but the surmicaccous enclaves may be restite in the Port Mouton Pluton and the South Mountain Batholith; in the Barrington Passage tonalite they probably represent melanosomes after incorporated xenoliths. Microgranular and coarse-grained granitoid enclaves apparently represent stoped autoliths of both quenched and slowly cooled granitic melt in the multiply-intrusive granitoid bodies.
RÉSUMÉ
Quatre intrusions granitiques du Dévonien supérieur dans la zone de Meguma, dans le sud-ouest de la Nouvelle-Écosse, renferment des enclaves abondantes typiques de masses granitiques orogéniques. Les intrusions ignées de Barrington Pas sage et de Shelbume renferment un assemblage d'enclaves cornéennes métasédimentaires granoblastiques (49 %) comprenant des enclaves surmicacles et porphyroblastes d'aluminosilicates (51 %) constituées de moins de 70 % de biotitc entrecroisée d'inclusions d'apatite et de zircon. Les enclaves métasédimentaircs prédominent dans le pluton de Port Mouton et le batholite de South Mountain (52 %), mais ces intrusions renferment en outre des enclaves granitiques microgrenues et a gros grains (25 % et 23 % respectivement) comportant des assemblages minéraux hyperalumineux et dotées de compositions tonalitiques à leucomonzogranitiques. Des concentrations élevées d'enclaves métasédimentaires aux surfaces de contact encaissantes laissent supposer qu'elles se sont probablement formées en tant que xinolites abattus du groupe de Meguma. Aucun xéiolite ne correspond aux paléosomes du gneiss du socle provenant du protolite des magmas granitiques, mais les enclaves surmicacees pourraient etre des melanosomes dans le pluton de Port Mouton et le batholite de South Mountain. Dans la tonalite de Barrington Passage, dies représented probablement des melanosomes apparus apres l'incorporation des xénolites. Les enclaves granitiques microgrenues et à gros grains represented apparemment des enclaves syngénétiques de magma granitique tant figéque douce men t refroidi dans les masses granitiques intrusives multipli.
[Traduit par la rédaction]
Research Article| April 01, 1999 Structure and emplacement history of a multiple-center, cone-sheet–bearing ring complex: The Zarza Intrusive Complex, Baja California, Mexico S. E. Johnson; S. E. Johnson 1Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia2Departamento de Geología, CICESE, Km 107 Carratera, Ensenada-Tijuana, Baja California, México Search for other works by this author on: GSW Google Scholar S. R. Paterson; S. R. Paterson 3Department of Earth Sciences, University of Southern California, California 90089-0740 Search for other works by this author on: GSW Google Scholar M. C. Tate M. C. Tate 1Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information S. E. Johnson 1Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia2Departamento de Geología, CICESE, Km 107 Carratera, Ensenada-Tijuana, Baja California, México S. R. Paterson 3Department of Earth Sciences, University of Southern California, California 90089-0740 M. C. Tate 1Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1999) 111 (4): 607–619. https://doi.org/10.1130/0016-7606(1999)111<0607:SAEHOA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation S. E. Johnson, S. R. Paterson, M. C. Tate; Structure and emplacement history of a multiple-center, cone-sheet–bearing ring complex: The Zarza Intrusive Complex, Baja California, Mexico. GSA Bulletin 1999;; 111 (4): 607–619. doi: https://doi.org/10.1130/0016-7606(1999)111<0607:SAEHOA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Cretaceous Zarza Intrusive Complex, located in the Peninsular Ranges of Baja California Norte, Mexico, is perhaps the best-preserved multiple-center, cone-sheet–bearing ring complex documented in North America. The 7 km2 elliptical complex hosts three nested, non-concentric intrusive centers that are successively younger to the south. The northern and central centers show the same evolutionary sequence of (1) intrusion of concentric gabbroic cone sheets, (2) intrusion of massive core gabbros, and (3) development of subvertical, ductile ring faults. Ring-fault kinematics indicate that both centers moved down relative to the surrounding country rocks, suggesting collapse into an underlying magma chamber. The southern center is composed of approximately equal proportions of gabbro and tonalite and lacks cone sheets. Aluminum-in-hornblende barometry on the tonalite indicates a maximum emplacement depth of 2.3 ± 0.6 kbar. The Zarza Intrusive Complex is surrounded by a ductile deformation aureole, and bedding is inward dipping and inward younging around the entire complex. Excellent preservation of the intrusive history allowed us to evaluate the origin of the aureole, and the three most applicable models are (1) collapse of the complex into its underlying magma chamber, (2) sinking of the complex and its chamber after solidification, and (3) formation of the aureole prior to emplacement of the complex. The preserved structural and intrusive relationships provide information on the dynamic evolution of subvolcanic magma chambers and suggest that the complex may have been overlain by a caldera. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Late Devonian (377–368 Ma, 40 Ar/ 39 Ar; 376 Ma, U–Pb) mafic intrusions in the Meguma Zone crop out as dykes, plugs, and synplutonic bodies of gabbro, diorite, or lamprophyre. All of the intrusions have similar lithologie characteristics and hydrous ferromagnesian mineral assemblages, and they appear to represent a genetically related series of mafic bodies with similar petrogenetic histories in the crust of the Meguma Zone. The intrusions show wide chemical variation of SiO 2 (45.7–65.7 wt.%), Al 2 O 3 (8.9–26.5 wt.%), MgO (2.8–26.5 wt.%), CaO (1.2–11.2 wt.%), and K 2 O (0.1–4.4 wt.%), and they have calc-alkaline, high-K calc-alkaline, and shoshonitic characteristics. Large-ion lithophile elements (LILE) are present at variable but high concentrations (e.g., Ba = 62–1920 ppm, Sr = 176–2567 ppm) relative to most high field strength element (HFSE) abundances (e.g., Y = 10–37 ppm, Zr = 8–421 ppm), and light rare-earth elements (LREE) have much higher concentrations than heavy rare-earth elements (HREE) (La/Lu = 24–330). Initial Sr isotopic ratios (0.7044–0.7079) and ε Nd values (−4.36 to 3.69) are highly variable. Scatter on major oxide variation diagrams probably results from the fractionation of all the major modal phases in the intrusions (olivine, augite, hornblende, and (or) plagioclase), and the cumulate characteristics of some bodies support this suggestion. Nevertheless, parallel patterns for the intrusions on mid-ocean ridge basalt (MORB) normalized spider diagrams support the notion of similar mafic parent melts, and Sr–Nd isotopic data identify contamination by continental crust in only one of the intrusions. The most primitive picrite contains approximately basaltic HFSE in conjunction with HREE at 5–11 CN , perhaps suggesting that the magmas emanated from depleted peridotite or pyroxenite, but high alkalies, LILE (<60 MN ), and LREE (10–100 CN ), and elevated initial Sr ratios in all of the intrusions, also require the existence of an enriched source component. Troughs in the spider diagrams at Ta, Nb, and Ti, and Sr–Nd isotopic values comparable with modern island-arc basalts, suggest that fluids derived from subducted ocean lithosphère metasomatized the mantle. Tectono-magmatic discriminators imply a continental margin arc environment rather than an island arc, and the intrusions record either Early Devonian subduction of Iapetus Ocean lithosphère beneath the Avalon terrane, Middle Devonian subduction of Theic Ocean lithosphère beneath the Meguma terrane, or an inherited subduction signature formed during a much older event.
The Early Cretaceous (∼115 Ma) Zarza Intrusive Complex is a small (<10 km2), bimodal ring complex that may represent a magmatic microcosm of the western Peninsular Ranges batholith. Its tholeiitic gabbro bosses (25% by area; Al2O3 > 17 wt %, Sr < 463 ppm) formed at subvolcanic depths <0.2 GPa (8 km) by >30% plagioclase accumulation from andesitic magma batches now preserved as cone-sheets (63%; SiO2 ≥ 55%, MgO < 3.1%, Ni ∼30 ppm). Quenched cone-sheets are polymorphic (olivine–pyroxene- or hornblende-bearing) and share similar chemical and isotopic compositions (εNd +7, 87Sr/86Srt = 115 < 0.704) that preclude extensive sediment contamination. Their calc-alkaline basalt parents apparently contained very different volatile concentrations (∼3–7 wt % H2O) inherited from various equilibria between subduction-related aqueous fluids, and depleted lherzolite in the upper mantle. Recharge and/or dominant ferromagnesian mineral fractionation at ∼0.8 GPa (>28 km) depth best explains subsequent differentiation towards high-Al andesite. Contemporaneous tonalite (SiO2 64–74%, molar Al2O3/(CaO + Na2O + K2O) [A/CNK] > 1.0, 87Sr/86Sri 0.703) probably formed in situ by andesite fractionation, whereas spatially associated trondhjemite (A/CNK > 0.98, 87Sr/86Sri 0.702) is more consistent with 8–19% dehydration melting of metabasite in the contact aureole. Enrichments of incompatible K2O, Ba, Rb and Th in all silica-saturated rocks from the western part of the batholith can be explained by mixing between different proportions of fractionated and partially melted end-members generated within thick oceanic arc basement.
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