Nd and Sr isotopic constraints on the petrogenesis of the west side of the northern Coast Mountains batholith, Alaskan and Canadian Cordillera

1991 ◽  
Vol 28 (6) ◽  
pp. 939-946 ◽  
Author(s):  
Scott D. Samson ◽  
P. Jonathan Patchett ◽  
William C. McClelland ◽  
George E. Gehrels
Keyword(s):  
Late Cretaceous ◽  
Crustal Thickening ◽  
Northern Coast ◽  
Crustal Material ◽  
The West ◽  
Crustal Component ◽  
Coast Mountains ◽  
Early Proterozoic ◽  
Depleted Mantle ◽  
Wrangellia Terrane

Nd and Sr isotopic ratios are reported from 15 samples of plutons of the northern Coast Mountains batholith (CMB), between. the Alexander–Wrangellia terrane and the Stikine terrane of southeastern Alaska. Samples of plutons that are part of the Late Cretaceous – Eocene CMB suite have a range in initial εNd of −3.0 to −0.2 and 87Sr/86Sr of 0.70494–0.70607. There is no correlation of isotopic ratio with age, lithology, or geographic location of these plutons. Two plutons that are probably older than the bulk of the CMB plutons have present-day εNd values of −6.8 and −2.6.The Late Cretaceous – Eocene plutons have Nd depleted-mantle model ages (tDM) of 620–1070 Ma. These data indicate that the northern CMB must contain a significant component of old, evolved continental crust. The presence of an old crustal component is further demonstrated by inherited zircons of average Early Proterozoic age contained in some plutons. The mid to Late Proterozoic tDM ages of the CMB plutons are therefore a result of a mixture of Early Proterozoic crustal material with. younger, juvenile crust. The most likely source of this old crustal component is the Yukon–Tanana terrane, a fragment composed of ancient crustal material that occurs within and directly to the west of the northern CMB. The juvenile component is probably a combination of material derived from the mantle and from anatexis of the surrounding juvenile terranes. Crustal anatexis may have occurred as a result of the intrusion of mafic melts related to subduction along the outboard margin of the Alexander–Wrangellia terrane, by crustal thickening due to the underthrusting of the Alexander–Wrangellia terrane beneath the Yukon–Tanana and Stikine terranes, or by a combination of both processes.

U–Pb geochronology of Late Cretaceous and early Tertiary plutons in the northern Coast Mountains batholith

1991 ◽  
Vol 28 (6) ◽  
pp. 899-911 ◽  
Author(s):  
George E. Gehrels ◽  
William C. McClelland ◽  
Scott D. Samson ◽  
P. Jonathan Patchett ◽  
David A. Brew
Keyword(s):  
Shear Zone ◽  
Late Cretaceous ◽  
Detrital Zircons ◽  
Northern Coast ◽  
Coast Mountains ◽  
Western Margin ◽  
Early Proterozoic ◽  
Isotopic Signatures ◽  
Metasedimentary Rocks ◽  
Early Tertiary

U–Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic–granodioritic bodies in central portions of the batholith yield ages of 59–58 Ma, coeval with younger phases of the tonalitic sheets. Large granite–granodiorite bodies in central and eastern portions of the batholith were emplaced at 51–48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Variations in the age, shape, location, and degree of fabric development among the various plutons indicate that Late Cretaceous – Paleocene tonalitic bodies were emplaced into a steeply dipping, dip-slip shear zone that was active along the western margin of the batholith. Postkinematic Eocene plutons were emplaced at shallow crustal levels. Inherited zircon components in these plutons range in age from mid-Paleozoic to Early Proterozoic and are coeval with detrital zircons in adjacent metasedimentary rocks. These old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons.

Age and petrogenesis of the Qassiarsuk carbonatite-alkaline silicate volcanic complex in the Gardar rift, South Greenland

1997 ◽  
Vol 61 (407) ◽  
pp. 499-513 ◽  
Author(s):  
Tom Andersen
Keyword(s):  
Volcanic Rocks ◽  
Single Stage ◽  
Volcanic Complex ◽  
Crustal Material ◽  
Crustal Component ◽  
Depleted Mantle ◽  
Common Parent ◽  
Nd Model Age ◽  
Extrusive Carbonatite ◽  
Age Estimates

AbstractThe Qassiarsuk (formerly spelled Qagssiarssuk) complex is located in a roughly E–W trending graben structure between Qassiarsuk village and Tasiusaq settlement in the northern part of the Precambrian Gardar rift, South Greenland. The complex comprises a sequence of alkaline silicate tuffs and extrusive carbonatites interlayered with sandstones, and their subvolcanic equivalents, which represent possible feeders for the extrusive rocks. The Rb-Sr, Sm-Nd and Pb isotopic characteristics of 65 samples of extrusive carbonatite- and silicate tuffs and carbonatite diatremes have been determined by mass spectrometry. The Qassiarsuk complex can be dated to c. 1.2 Ga by Rb-Sr and Pb-Pb isochrons on whole-rocks and mineral separates, agreeing with previous isotopic ages for the volcanic rocks of the Eriksfjord formation in the Eriksfjord area of the Gardar rift, but not with previous, indirect age estimates of >1.31 Ga for assumed Eriksfjord equivalents in the Motzfeldt area further east. Recalculated isotopic compositions at 1.2 Ga indicate that the Qassiarsuk carbonatite- and alkaline-silicate magmas were comagmatic and derived from a depleted mantle source (εNd>4, εSr<−13, time-integrated, single- stage 238U/204Pb ≤ 7.4). The mantle-derived magmas were contaminated with crustal material, equivalent to the local, pre-Gardar granites and gneisses and sediments derived from these. The crustal component has a depleted mantle Nd model age of 2.1-2.6 Ga; at 1.2 Ga it was characterized by εSr = +76, εNd = −8.4, time-integrated, single- stage 238U/204Pb = 8.2−8.3. Strong decoupling of the Pb from the Sr and Nd isotopic systems suggests that the contamination happened only after carbonatitic and alkaline-silicate magmas had evolved from a common parent, by processes such as liquid immisicibility and/or fractional crystallization. Post-magmatic hydrothermal alteration (oxidation, hydration of mafic silicates, carbonatization of melilite) may have contributed further to the contamination of the carbonatite and alkaline silicate rocks of the Qassiarsuk complex.

scholarly journals Late Cretaceous to Paleocene Tectonometamorphic Evolution of the Blanchard River Assemblage, Southwest Yukon: New Insight into the Terminal Accretion of Insular Terranes in the Northern Cordillera

Lithosphere ◽  
2020 ◽  
Vol 2020 (1) ◽  
Author(s):  
Lianna Vice ◽  
H. Daniel Gibson ◽  
Steve Israel
Keyword(s):  
British Columbia ◽  
Late Cretaceous ◽  
Amphibolite Facies ◽  
Contact Metamorphism ◽  
Northern Coast ◽  
Coast Mountains ◽  
Tectonic Events ◽  
Western Cordillera ◽  
History Of ◽  
Northwestern North America

Abstract The Intermontane-Insular terrane boundary stretches over 2000 kilometers from British Columbia to Alaska in the western Cordillera. Juxtaposed between these terranes is a series of Jura-Cretaceous basinal and arc assemblages that record a complicated and contested tectonic evolution related to the Mesozoic-Paleocene accretionary history of northwestern North America. In southwest Yukon, west-verging thrust faults facilitated structural stacking of the Yukon-Tanana terrane over these basinal assemblages, including the Early Cretaceous Blanchard River assemblage. These previously undated compressional structures are thought to be related to the final collapse of the Jura-Cretaceous basins and the tectonic burial of the Blanchard River assemblage resulting in amphibolite facies metamorphism. New in situ U-Th-Pb monazite ages record at least three tectonic events: (1) the tectonic burial of the Blanchard River assemblage to amphibolite facies conditions between 83 and 76 Ma; (2) peak burial was followed by regional exhumation at ca. 70-68 Ma; and (3) intense heating and ca. 63-61 Ma low-pressure contact metamorphism attributed to the intrusion of the voluminous Ruby Range suite, which is part of the northern Coast Mountains batholith. The tectonometamorphic evolution recorded in the Blanchard River assemblage can be correlated to tectonism within southwest Yukon and along the length of the Insular-Intermontane boundary from western British Columbia through southwestern Yukon and Alaska. In southwest Yukon, these results suggest an asymmetric final collapse of Jura-Cretaceous basins during the Late Cretaceous, which relates to the terminal accretion of the Insular terranes as they moved northward.

Thermobarometric constraints on the structural evolution of the Coast Mountains batholith, central southeastern Alaska

1991 ◽  
Vol 28 (6) ◽  
pp. 912-928 ◽  
Author(s):  
William C. McClelland ◽  
Lawrence M. Anovitz ◽  
George E. Gehrels
Keyword(s):  
Shear Zone ◽  
Late Cretaceous ◽  
Structural Evolution ◽  
Fault System ◽  
Northern Coast ◽  
Coast Mountains ◽  
Petersburg Region ◽  
Temperature And Pressure ◽  
Gravina Belt ◽  
Zone Temperature

Thermobarometric data from amphibolite-facies metamorphic rocks west of the Coast Mountains batholith provide important constraints on the structural evolution of the mid-Cretaceous Sumdum–Fanshaw fault system and Late Cretaceous – Paleocene Le Conte Bay shear zone in central southeastern Alaska. Ductile structures that make up the Sumdum–Fanshaw fault system record the east-directed underthrusting of the Alexander terrane and Gravina belt beneath the Ruth assemblage (Yukon–Tanana terrane) and Taku terrane. These structures are truncated to the east by the Le Conte Bay shear zone. Temperature and pressure estimates calculated from the garnet–biotite geothermometer and garnet–rutile–ilmenite–plagioclase–quartz geobarometer suggest juxtaposition of the Gravina belt and Yukon–Tanana terrane at relatively deep levels (>7 kbar) during mid-Cretaceous time. Rocks west of the Le Conte Bay shear zone yield thermobarometric estimates of 465–890 ± 50 °C and 7.1–11.8 ± 1 kbar (1 kbar = 100 MPa). Late Cretaceous and Paleocene metamorphism associated with the Le Conte Bay shear zone reflects synkinematic emplacement of tonalitic intrusions along the western margin of the Coast Mountains batholith. Thermobarometric results from samples adjacent to the tonalite bodies record uplift and retrogression and suggest tonalite emplacement at 7.5–7.7 ± 1 kbar. An eastward increase in thermobarometric estimates observed in Thomas and Le Conte bays is inferred to record uplift and east-side-up tilting of rocks west of and within the Le Conte Bay shear zone during Late Cretaceous and Paleocene time. Rocks within the Le Conte Bay shear zone were apparently rapidly (1.5–2 mm/a) uplifted to shallow crustal levels prior to mid-Eocene time. Thermobarometric results for the Petersburg region are similar to those previously reported along the western flank of the northern Coast Mountains batholith.

scholarly journals New zircon radiometric U-Pb ages and Lu-Hf isotopic data from the ultramafic-mafic sequences of Ranau and Telupid (Sabah, eastern Malaysia): Time to reconsider the geological evolution of Southeast Asia?

Geology ◽  
2021 ◽  
Author(s):  
Basilios Tsikouras ◽  
Chun-Kit Lai ◽  
Elena Ifandi ◽  
Nur’Aqidah Norazme ◽  
Chee-Hui Teo ◽  
...  
Keyword(s):  
Ultramafic Rocks ◽  
Crustal Material ◽  
Isotopic Data ◽  
Backarc Basin ◽  
Crustal Component ◽  
Sulu Sea ◽  
Potential Source ◽  
Depleted Mantle ◽  
Geological Evolution ◽  
Mantle Reservoir

New zircon U-Pb geochronology from a peridotite suite near Ranau and the Telupid ophiolite in Sabah, eastern Malaysia, contradict previous studies, which assumed that the Sabah mafic-ultramafic rocks are largely ophiolitic and Jurassic–Cretaceous in age. We show that these rocks formed during a magmatic episode in the Miocene (9.2–10.5 Ma), which is interpreted to reflect infiltration of melts and melt-rock reaction in the Ranau subcontinental peridotites during extension, and concurrent seafloor spreading forming the Telupid ophiolite further south. Older zircons from the Ranau peridotites have Cretaceous, Devonian, and Neoproterozoic ages. Zircon Lu-Hf isotopic data suggest their derivation from a depleted mantle. However, significant proportions of crustal components have been incorporated in their genesis, as evidenced by their less-radiogenic Hf signature compared to a pristine mantle reservoir. The involvement of a crustal component is consistent with our interpreted continental setting for the Ranau peridotite and formation in a narrow backarc basin for the Telupid ophiolite. We infer that the Sulu Sea, which was expanding throughout much of the Miocene, may have extended to the southwest into central Sabah. The Telupid oceanic strand formed during the split, collapse, and rollback of the Sulu arc due to the subduction of the Celebes Sea beneath Sabah. Incorporation of the Sulu arc in the evolving Miocene oceanic basin is a potential source to explain the involvement of crustal material in the zircon evolution of the Telupid ophiolite.

Contrasting tectono-stratigraphy and structure in the Coast Belt near Chilko Lake, British Columbia: unrelated terranes or an arc – back-arc transect?

1994 ◽  
Vol 31 (11) ◽  
pp. 1700-1713 ◽  
Author(s):  
Paul J. Umhoefer ◽  
Margaret E. Rusmore ◽  
G. J. Woodsworth
Keyword(s):  
Late Cretaceous ◽  
Structural Evolution ◽  
Fault System ◽  
The West ◽  
Volcanic Arc ◽  
Coast Mountains ◽  
Magmatic Arc ◽  
Clastic Rocks ◽  
Oblique Convergent ◽  
Back Arc

Stratigraphy and structural styles vary greatly in two areas of the Coast Belt near Chilko Lake (Chilcotin Ranges in the east and Coast Mountains in the west). No definite continuity between the two belts has been established in the pre-mid-Cretaceous geology, but this area may be a long-lived, episodic magmatic arc and nearby arc-related basin. The stratigraphic contrasts may reflect inherent differences between an arc and related basinal sequence. Triassic volcanic-arc sequences are part of the Stikine (western belt) and Cadwallader (eastern belt) terranes, which may be part of the same arc. The Jurassic is represented by one dated pluton in the west compared with almost continuous deposition of volcanogenic clastic rocks in the east. Lower Cretaceous sequences in the west and east may represent a volcanic arc and back-arc basin. The Taylor Creek Group (Albian) is the first definitive link between the two belts and represents an arc and intra-arc or back-arc basin. The structural evolution of the two belts also differs significantly. The early Late Cretaceous Eastern Waddington thrust belt comprises all major structures in the west, but only has minor expression in the east. Most of the structures in the east are part of the latest Cretaceous(?) to early Tertiary dextral-strike-slip, Yalakom fault system. These differences were most likely caused by the Late Cretaceous change from nearly orthogonal subduction to a dextral-oblique convergent margin.

Late Cretaceous – early Paleogene tectonic evolution of the Central Pamir inferred from the geochemical features of the Bartang volcanics

2020 ◽  
Author(s):  
Jovid Aminov ◽  
Guillaume Dupont-Nivet ◽  
Lin Ding ◽  
Stephane Guillot ◽  
Johannes Glodny ◽  
...  
Keyword(s):  
Late Cretaceous ◽  
Tectonic Setting ◽  
Isotope Ratios ◽  
Igneous Rocks ◽  
Crustal Thickening ◽  
Crustal Material ◽  
Magmatic Activity ◽  
Volcanic Suite ◽  
Early Paleogene ◽  
Related Pattern

&lt;p&gt;The Pamir orogen in Central Asia has formed by the amalgamation of several Gondwana-derived terranes and their accretion to the southern Eurasian margin in the Mesozoic. Later on, the crust of the Pamir orogen was strongly deformed and uplifted as a result of the Cenozoic India-Asia collision. The deformation of the Pamir orogen, which resulted in shortening, crustal thickening and exhumation of deep crustal rocks within the gneiss domes of the Central and Southern Pamir makes the area an ideal site for studying the India-Asia collision and its paleogeographic and climatic effects. To account for today&amp;#8217;s 70-km-thick crust of the Pamir orogen and more than 400 km of convergence accommodated in the Pamir, pre- and syn-collisional processes have been proposed including, continental subduction, delamination, extrusion and oroclinal bending of the Pamir arc. However, testing these models requires constraints on the pre-collisional state of the Pamir lithosphere and its tectono-magmatic evolution. During most of the Cretaceous, the southern Pamir terrane was a site of a widespread arc-related magmatism, which resulted in the formation of many plutons and a volcanic suite of intermediate to acidic composition, whereas the central Pamir terrane lacked any sign of magmatic activity. However, in the late Cretaceous to early Paleogene (78 &amp;#8211; 61 Ma) a less widespread magmatic activity in the western part of the Central Pamir resulted in the formation of the Bartang mafic to intermediate volcanic and volcaniclastic rocks. We report here the geochemical and Sr-Nd isotopic features of the late Cretaceous &amp;#8211; early Paleogene Bartang volcanics. This volcanic suite bears geochemical and radiogenic isotope features that differ from the arc-related southern Pamir igneous rocks. Mafic basalts that comprise the lowest portion of the section exhibit MORB-like pattern with slightly depleted light rare earth elements (LREE) and large ion lithophile elements (LILE). Further up in the section this pattern shifts towards an arc-related pattern with enriched LREE and LILE. The 87Sr/86Sr&lt;sub&gt;i&lt;/sub&gt; isotope ratios are lower (0.705335 &amp;#8211; 0.706693) than those from the southern Pamir igneous rocks (0.706915 &amp;#8211; 0.711105) and epsilon Nd values exhibit ratios close to mantle domain, ranging between -0.7 and -2.7, with the lower part of the section showing less negative values then the upper. In contrast to the Bartang volcanics, the southern Pamir igneous rocks exhibit more negative epsilon Nd values (from -4.7 to -13). The relatively low initial 87Sr/86Sr isotope ratios and slightly negative epsilon Nd values of the Bartang volcanic rocks together with the trace elements pattern that shifts from MORB-like to arc-related indicate mixing of two magmas derived from depleted and enriched mantle sources, with the latter inheriting the arc-related pattern from the subduction stage. Alternatively, the arc-related pattern could be derived through contamination of the primary magma by the crustal material. These features, compared to the southern Pamir arc-related igneous rocks, also indicate that the tectonic setting in the Pamir changed during the late Cretaceous from a continental arc to a within-plate extensional setting.&lt;/p&gt;

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