Jurassic Continental Arc Research Articles

U–Pb zircon geochronology and geochemistry of the Jurassic magmatic rocks from the region of Cananea and Nacozari, northeastern Sonora, Mexico: timing and composition of the southernmost edge of the Jurassic continental arc

Igneous rocks of the Jurassic continental arc of southwest North American Cordillera crop out in the region between Cananea and Nacozari, in northeastern Sonora, Mexico. Contemporaneous igneous rocks occur farther south in the Nazas rift province; however, their tectonic affinity is debated between the southern extension of the Cordilleran arc and an origin associated with decompressive mantle melting during the Pangea breakup. We consider that the studied rocks represent the southernmost expression of the Jurassic continental arc. Four magmatic units are distinguished based on geology, U–Pb zircon geochronology, and geochemical composition. They comprise the Buenos Aires granite dated at 183.8 ± 1.1 Ma, the Mababi granite with ages of 175.5 ± 1.8, 173.5 ± 1.8, and 171.7 ± 1.0 Ma, the Elenita Formation volcanic rocks dated at 169.7 ± 1.7 Ma, and the Cerro Blanco granite dated at 162.3 ± 2.4 Ma. The studied samples correspond to highly differentiated rocks with high-silica contents (69–76 wt.%), low Mg# values (mostly <45), and alkali–calcic and peraluminous compositions suggesting crustal contamination. LREE-enriched normalized slopes with negative Eu anomalies and multielement plots characterized by well-developed negative anomalies of Nb–Ta, P, and Ti provide an unequivocal signal of the continental arc nature. In contrast to the Nazas province, where the Jurassic magmatism consists of discreet volcaniclastic sequences deposited in extensional basins with no evidence of plutonic rocks, the Jurassic continental arc of Arizona and northern Sonora forms a regionally coherent geological feature characterized by large exposures of volcanic and plutonic rocks.

The “Nazas Arc” is a continental rift province: Implications for Mesozoic tectonic reconstructions of the southwest Cordillera, U.S. and Mexico

Abstract We reject the notion of a Jurassic continental arc in eastern Mexico, termed the “Nazas arc,” on geologic grounds. Instead, we propose that the Jurassic continental arc of the SW Cordilleran U.S. and northern Sonora, Mexico, passed southward into the oceanic realm and is represented by Jurassic arc volcanic and plutonic rocks that fringed the Mexican paleo-Pacific margin, which are currently found in the western Peninsular Ranges of southern California, USA, and Baja California, the Vizcaino Peninsula of Baja California, and western mainland Mexico. To show this, we present a summary of the geologic features of a continental arc, using the geology of the southern end of the Jurassic continental arc, in southern Arizona and northern Sonora. These features include multi-kilometer–thick sections of volcanic rock; large volcanic centers, including silicic calderas; major eruptive units that can be correlated for distances of 100 km or more; abundant, large plutonic suites; and continuity of these features for distances of hundreds of kilometers along the length of the continental arc. Then we show that the “Nazas arc” consists of scattered, small continental rift basins with thin (meters to tens of meters thick) volcanic sections at the base of clastic sections that are hundreds of meters thick. Plutonic rocks are entirely absent from the “Nazas arc,” despite the fact that post-Jurassic tectonic events should have exposed them if they existed. This paper also presents a tabulation of all published U-Pb zircon dates in the Jurassic continental arc of southern Arizona, USA, and northern Sonora (Table 1A), and in the “Nazas arc” of eastern Mexico (Table 1B), with ages, methods, the rock type dated, and notes on geologic relations. We use this to detail the abundance of thick, laterally extensive volcanic sections and large plutonic suites in a continental arc (the Jurassic arc of southern Arizona–northern Sonora), which contrasts sharply with the “Nazas arc.” The term “Nazas arc” has been in widespread usage for volcanic rocks in eastern Mexico for decades in many dozens of papers, and it is portrayed as a 2000-km-long, 250-km-wide belt that extends from Sonora through eastern Mexico to Chiapas. It has been misunderstood to form a subduction-related silicic large igneous province (SLIP), and it has been proposed that the Gulf of Mexico formed as a backarc basin behind the “Nazas arc.” The “Nazas arc” model also requires an east-dipping subduction zone under Mexico, and a separate west-dipping subduction zone under the oceanic arc rocks of western Mexico, which those models portray as an exotic arc, despite the presence of abundant detrital zircon from the Mexican margin. We urge workers to abandon the term “Nazas arc” and replace it with “Nazas rift province,” which represents continental rift basins formed during the breakup of Pangea.

Evolution of continental arc at root caused by igneous garnet and amphibole fractionation: Evidence from Jurassic intermediate–felsic intrusive rocks in southern Qiangtang, Tibet

Jurassic continental arc magmatism occurred widely across the southern Qiangtang Terrane, and provides an opportunity to study how continental arc magmas evolve towards intermediate–felsic compositions. We present new LA–ICP–MS zircon U–Pb ages, whole-rock major and trace element compositions, and Sr–Nd and in-situ zircon Hf isotope data for late Jurassic intermediate–felsic intrusive rocks from the Rena Co area of southern Qiangtang, central Tibet. Jurassic intrusive rocks in this area consist mainly of granodiorite, diorite, and related gabbro–diorite mafic enclaves, which have been dated at 153.1–147.6Ma. Combining our data with previous results shows that continental arc magmatism in the southern Qiangtang Terrane has relatively homogeneous Sr–Nd isotopic compositions, with (87Sr/86Sr)i ratios of 0.7069–0.7092 and εNd(t) values of −6.3 to −2.2. Assuming that the hot, hydrous, basaltic melt generated by the subduction of the oceanic slab has a composition similar to island arc basalts, mixing ~50% basaltic magma with ~50% lower crustal material can produce the Sr–Nd isotopic compositions of the Rena Co intrusive rocks. The southern Qiangtang arc magmatism also has similar Hf isotopic compositions, albeit with a large range of εHf(t) values (−10 to +3). The different Rena Co intrusive rocks have different geochemical characteristics, including major element and rare earth element (REE) contents, Mg#, and evolutionary trends on Harker diagrams. This suggests that these arc rocks were derived from a common parent magma, but experienced different magmatic processes. The diorites were derived from primitive arc magma through amphibole-dominated low-pressure fractional crystallization, whereas the granodiorites underwent garnet-dominated, high-pressure fractional crystallization in secondary magma chambers. Fractionation of igneous garnet and amphibole played a significant role in the evolution of the continental arc, and was likely an important mechanism in driving the continental arc towards silica-rich compositions. The fractionation of igneous garnet and amphibole possibly changes the Mg# and Nb/Ta ratios of the derivative magmas.

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.

Zircon U–Pb dating and whole‐rock geochemistry of volcanic rocks in eastern Heilongjiang Province, NE China: Implications for the tectonic evolution of the Mudanjiang and Paleo‐Pacific oceans from the Jurassic to Cretaceous

The eastern Heilongjiang Province, NE China, is located in the eastern CAOB (Central Asian Orogenic Belt). Previous studies suggest that subduction of the Mudanjiang and Paleo‐Pacific oceans had been initiated in this area during the early Mesozoic. However, research on the Mesozoic volcanic rocks is insufficient, limiting our understanding of the relationship between magmatism and subduction process. To solve the problem, seven volcanic rocks in the Lesser Xing'an‐Zhangguangcai Range and the Jiamusi Block were collected for zircon U–Pb dating (SHRIMP) and whole‐rock geochemical analysis. Most samples from the Lesser Xing'an‐Zhangguangcai Range are Jurassic in age (189–178 Ma), including andesite, trachyandesite, trachydacite, and rhyolite. The andesitic to trachytic rocks were generated by magma mixing at an active continental margin, whereas the rhyolite is highly fractionated I‐type. All the samples from the Jiamusi Block and one basaltic andesite from the Lesser Xing'an Range are late Early Cretaceous in age (ca. 100 Ma) and were formed in a back‐arc extensional setting (roll‐back). Fractional crystallization was important in the generation of most Cretaceous rocks (basalt to trachydacite). However, magma mixing or crustal contamination was required to explain some andesites from the Jiamusi Block showing extremely low Cr, Ni, and Mg#. When integrated with the coeval blueschist‐facies metamorphism, the Early Jurassic volcanism indicates an active continental margin in the Lesser Xing'an‐Zhangguangcai Range due to the westward subduction of the Mudanjiang Ocean. After the closure of the Mudanjiang Ocean, two NE/NNE trending sinistral strike‐slip faults in NE China were triggered, accompanied by the slab roll‐back of the Paleo‐Pacific oceanic plate. The Jurassic continental arc and accretionary complexes in NE China were cut through by the two strike‐slip faults and bended to form small oroclines. At ca. 100 Ma, numerous accretionary complexes were accreted along the eastern margin of the CAOB, further forcing the retreat of trench and slab roll‐back. The late Early Cretaceous intra‐continental volcanism was generated in the Lesser Xing'an‐Zhangguangcai Range and Jiamusi Block during this roll‐back process.

Volcanic sequence in Late Triassic – Jurassic siliciclastic and evaporitic rocks from Galeana, NE Mexico

In northeastern Mexico, volcanic rocks interbedded with Late Triassic–Jurassic siliciclastic and evaporitic strata have been linked to magmatic arcs developed in the Pangea western margin during its initial phase of fragmentation. This work provides new petrographic and geochemical data for volcanism included in the El Alamar and Minas Viejas formations outcropping in the Galeana region. Andesitic dykes and sills (n= 10) in the El Alamar redbeds show SiO2= 47.5–59.1% and MgO= 1.2–4.2%, as well as a geochemical affinity to island arc magmas. This work represents the first report of this tectonic setting in the region. Geological and petrographic evidence suggest that this arc system likely developed after ~220 and before ~193Ma. Trachy-andesitic and rhyodacitic domes (n= 20) associated with the Minas Viejas gypsum-carbonates sequence show SiO2= 61.8–82.7% and MgO= 0.1–4.0% with a tectonic affinity to continental arc. A rhyodacite sample from this region has been dated by U-Pb in zircon, yielding an age of 149.4 ± 1.2Ma (n= 21), being the youngest age related to this arc. Finally, we propose a threestep model to explain the tectonic evolution from Late Triassic island arc to Jurassic continental arc system in the northeastern Mexico.

Stratigraphy and age of Upper Jurassic strata in north-central Sonora, Mexico: Southwestern Laurentian record of crustal extension and tectonic transition

Stratigraphy, sedimentology, and geochronology of the Upper Jurassic Cucurpe Formation in north-central Sonora, Mexico, provide new insights into Late Jurassic rifting along the southwestern margin of Laurentia. The Cucurpe Formation is the fill of the Altar-Cucurpe Basin. This basin developed upon attenuated crust of the Triassic–Middle Jurassic continental arc and was part of the Arivechi-Cucurpe seaway; a narrow marine embayment oriented parallel to and located west of the Chihuahua trough. The Cucurpe Formation unconformably overlies Middle Jurassic arc assemblages and represents upward-coarsening marine prodeltaic deposits. New U-Pb zircon geochronology and a Kimmeridgian ammonite ( Idoceras cf. I . densicostatum ) constrain its age to between ca. 158 and 149 Ma. Detrital zircon ages from the unconformably overlying Lower Cretaceous Bisbee Group indicate a maximum depositional age of 139 ± 2 Ma (2σ error), demonstrating a hiatus of at least 10 m.y. between Jurassic and Cretaceous strata. Detrital zircon ages and petrographic data indicate the provenance of Cucurpe Formation and lowermost Bisbee strata. The lower part of the Cucurpe was derived dominantly from Middle Jurassic volcano-sedimentary successions. The upper part of the Cucurpe Formation was largely derived from syneruptive volcanic material equivalent to the Ko Vaya volcanic suite of southern Arizona and northern Sonora. Lowermost Bisbee strata were derived from Middle Jurassic arc rocks and exhumed Caborcan basement, Paleozoic–Lower Jurassic sedimentary cover, and Lower Cretaceous intermediate volcanic rocks. Revised stratigraphy of the Cucurpe-Tuape region indicates that several conglomeratic units, formerly interpreted as Late Jurassic pull-apart basin deposits, are not of Late Jurassic age.

Characteristics of Jurassic Continental Arc Magmatism in South Korea: Tectonic Implications

Abstract Study of whole‐rock geochemical and isotope (Sr‐Nd) characteristics of Jurassic plutons from South Korea shows differences in chemical compositions that might reflect the Jurassic tectonic environments of East Asia. These plutons are dominated by granites and granodiorites with subordinate intermediate monzodiorite to diorite phases. All analyzed samples show depletions in Ta‐Nb, P, and Ti and enrichment in large ion lithophile elements, characteristic of high‐K calc‐alkaline I‐type granitoids of the active continental margin. Early Jurassic plutons fall into two groups: \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontC...

Metal Sources in Mineral Deposits and Crustal Rocks of Ecuador (1° N–4° S): A Lead Isotope Synthesis

Abstract Ecuador consists of terranes having both continental (Chaucha, Tahuin, Loja terranes) and oceanic (Macuchi, Alao, Salado terranes) affinity, which were accreted to the Amazon craton from Late Jurassic to Eocene. Four main magmatic arcs were formed by the subduction of the Farallon/Nazca plate since the Jurassic: a Jurassic continental arc on the western margin of the Amazon craton, a Jurassic island arc (Alao terrane), an early Tertiary island arc (Macuchi terrane), and a middle-late Tertiary continental arc encompassing the terranes of Macuchi, Chaucha, Tahuin, Loja, and Alao after complete assembly of the Ecuadorian crust. Mineral deposits formed during these magmatic arc activities include porphyry-Cu and gold skarn deposits in association with the Jurassic continental arc, polymetallic volcanic-hosted massive sulfide deposits (VHMS) in association with the Jurassic island arc of Alao, Au-Cu-Zn VHMS deposits in association with the early Tertiary island arc of Macuchi, and porphyry-Cu and precious-metal epithermal deposits in association with the middle-late Tertiary continental-arc magmatism on the newly assembled crust of Ecuador (Macuchi, Chaucha, Tahuin, Loja, and Alao terranes). In this study, we have compiled 148 new and 125 previously published lead isotope analyses on Paleozoic to Miocene metamorphic, intrusive, volcanic, and volcanosedimentary rocks, as well as on Jurassic to Miocene magmatic-related ore deposits of Ecuador. Lead isotope compositions of the magmatic rocks of the four main arc events derive from mixing of various sources including mantle, variably enriched by pelagic sediments and/or by a high 238U/204Pb component, and heterogeneous continental crust rocks. Lead isotope compositions of the Ecuadorian ore deposits display a broad range of values (206Pb/204Pb = 18.3–19.3, 207Pb/204Pb = 15.54–15.74, 208Pb/204Pb = 38.2–39.2), which is as large as the range previously reported for all magmatic-related ore deposits of the Central Andean provinces I and II combined. Ore deposits formed before complete assembly of the Ecuadorian crust through complete accretion of the several terranes (i.e., pre-Eocene) have lead isotope compositions overlapping those of the associated magmatic rocks, suggesting a largely magmatic origin for their lead. In contrast, post-assembly ore deposits (i.e., post-Eocene) have lead isotope compositions that only partly overlap those of the coeval magmatic rocks of the continental arc. In fact, several ore deposits have lead isotope compositions shifted toward those of the basement rocks that host them, suggesting that lead derives from a mixture of magmatic lead and basement-rock lead leached by hydrothermal fluids. Most Ecuadorian ores have high 207Pb/204Pb values (>15.55), suggesting a dominant continental crust or pelagic sediment origin of the lead. However, we caution against concluding that chalcophile metals (for example, Cu and Au) also have a continental crust origin. Ore deposits of the different terranes of Ecuador, irrespective of their age, plot in distinct isotopic fields, which are internally homogeneous. This suggests that lithologic factors had an important control on the lead isotope compositions. Ultimately, lead isotope compositions of the ore deposits of Ecuador mirror the isotopic compositions of the rocks of the host terranes and are consistent with the multiterrane nature of the Ecuadorian crust.

Timing of deformation and accretion of the Antimonio terrane, Sonora, from paleomagnetic data

At present, few paleomagnetic data exist to test tectonic models that link northern Mexico with the evolution of the southern Cordillera of North America. We present new paleomagnetic data for the Antimonio terrane in northwest Mexico. A moderate-inclination, dual-polarity yet secondary magnetization in Triassic-Jurassic strata of the Antimonio Formation and a primary remanence in Cretaceous igneous rocks (combined result: D = 338.9°, I = 57.4° α 95 = 5.6°, k = 34.8; N = 20 sites) are interpreted to indicate stability of the Antimonio terrane with respect to the craton since the Late Cretaceous. A more important result, however, is a shallow-inclination, dual-polarity magnetization in Antimonio Formation strata that fails a fold test ( D = 186.7°, I = −28.4°; α 95 = 7.6°, k = 28.2; N = 14 sites). We interpret the shallow-inclination magnetization as a secondary remanence acquired during accretion of the Antimonio terrane to the North American margin in the Middle or Late Jurassic, or during deformation within an essentially autochthonous Jurassic continental arc.