USGS Science⌕ Search
Home/Archive/USGS

USGS · 70019925

Effects of interaction between ultramafic tectonite and mafic magma on Nd-Pb-Sr isotopic systems in the Neoproterozoic Chaya Massif, Baikal-Muya ophiolite belt

Y. V. Amelin·E. Yu Ritsk·L.A. Neymark
View original source ↗Export metadata ↓

Abstract

Sm-Nd, Rb-Sr and U-Pb isotopic systems have been studied in minerals and whole rocks of harzburgites and mafic cumulates from the Chaya Massif, Baikal-Muya ophiolite belt, eastern Siberia, in order to determine the relationship between mantle ultramafic and crustal mafic sections. Geological relations in the Chaya Massif indicate that the mafic magmas were emplaced into, and interacted with older solid peridotite. Hand picked, acid-leached, primary rock-forming and accessory minerals (olivine, orthopyroxene, clinopyroxene and plagioclase) from the two harzburgite samples show coherent behavior and yield 147 Sm/ 144 Nd- 143 Nd/ 144 Nd and 238 U/ 204 Pb- 206 Pb/ 204 Pb mineral isochrons, corresponding to ages of 640 ± 58 Ma (95% confidence level) and 620 ± 71 Ma, respectively. These values are indistinguishable from the crystallization age of the Chaya mafic units of 627 ± 25 Ma (a weighted average of internal isochron Sm-Nd ages of four mafic cumulates). The Rb-Sr and Sm-Nd isotopic systems in the harzburgite whole-rock samples were disturbed by hydrothermal alteration. These alteration-related isotopic shifts mimic the trend of variations in primary isotopic compositions in the mafic sequence, thus emphasizing that isotopic data for ultramafic rocks should be interpreted with great caution. On the basis of initial Sr and Nd values, ultramafic and mafic rocks of the Chaya Massif can be divided into two groups: (1) harzburgites and the lower mafic unit gabbronorites withε Nd = +6.6 to +7.1 andε Sr = −11 to −16; and (2) websterite of the lower unit and gabbronorites of the upper mafic unit:ε Nd = +4.6 to +6.1 andε Sr = −8 to −9. Initial Pb isotopic ratios are identical in all rocks studied, with mean values of 206 Pb/ 204 Pb= 16.994 ± 0.023 and 207 Pb/ 204 Pb= 15.363 ± 0.015. The similarity of ages and initial isotopic ratios within the first group indicates that the isotopic systems in the pre-existing depleted peridotite were reset by extensive interaction with basaltic magma during formation of the mafic crustal sequence. The isotopic data agree with a hypothesized formation of the Chaya Massif in a suprasubduction-zone environment.

Explore related subjects

Earth and Planetary Science LettersArticle
90° N90° S · 180° W ← longitude → 180° E
Source-reported bounding extent: 53.54030739150022° to 59.18718526575666° latitude; 106.6552734375° to 116.43310546875° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Y. V. Amelin, E. Yu Ritsk, L.A. Neymark. 1997. Effects of interaction between ultramafic tectonite and mafic magma on Nd-Pb-Sr isotopic systems in the Neoproterozoic Chaya Massif, Baikal-Muya ophiolite belt. https://doi.org/10.1016/s0012-821x(97)00046-0

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Dynamic drainage reorganization in Eastern Tibet: Insights from the Yangtze River first bend

The modern drainage network of eastern Tibet is widely believed to have developed through a series of river capture and flow reversal events; however, the timing and mechanisms driving this reorganization remain contentious. Among these events, the river capture that formed the First Bend of the Yangtze River (YFB) stands out as both iconic and particularly debated. Here we present sedimentary provenance data from the Late Miocene–Quaternary Dali Basin, located south of the YFB, which indicate that a southward-flowing Jinsha River (i.e., the present-day upper Yangtze River) sourced sediment to the Dali basin at ∼7.4–6.4 Ma in a drainage configuration different from that of today. Because this interval postdates the initial establishment of a near-modern Jinsha River system prior to the Miocene, our results imply at least two discrete fluvial reorganizations occurred at the YFB—one preceding ∼7.4 Ma and another following ∼6.4 Ma. By integrating these findings with landscape evolution modeling, we infer that the initiation of rapid uplift of the Yulong-Haba Mountains and the Diancang Shan may have been responsible for these drainage reorganizations. These results underscore that Cenozoic drainage systems on the eastern Tibetan Plateau have evolved dynamically on a short timescale of ∼10 5 –10 6 -year, rather than remaining in a long-term stationary configuration on ∼10 7 -year timescales.

eastern Tibetan Plateau, first bend of the Yangtze↗
USGS2026

Patterns of rift basin development and the fidelity of the subsidence record: Insights through Bayesian modeling of rapid tectonic subsidence in a Rio Grande rift basin, Socorro, NM, U.S.A

Characterizing tectonic subsidence rates within depositional sequences provides direct insight into the driving mechanism(s) of accommodation in a basin. However, the temporal resolution of this record is often stymied by a lack of high-precision and high-resolution ages, which enable a more complete description of basin subsidence drivers. We explore the effect of high-precision and high-resolution ages in modeling accommodation for the Miocene La Jencia Basin of the central Rio Grande rift (RGR) and interpret driving mechanism behavior from these models (e.g., lithospheric thinning). We present a new geochronologic dataset of both laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) data and use these ages in Bayesian accommodation modeling. Models constrained by high-precision and high-accuracy TIMS ages yield peak tectonic subsidence rates exceeding 220 m/Myr, and an average Miocene subsidence of ∼120 m/Myr. While timing and magnitude vary, all models suggest two pulses of rapid Miocene tectonic subsidence, which we interpret to reflect basin-bounding fault movement. Prior to peak subsidence, there was an initial period of fault linkage and organization that occurred over <1–3 Myr that produced the basin-bounding La Jencia-Cerro Colorado fault zone. A comparison of published tectonic subsidence rates to those modeled here shows that while tectonic subsidence during continental rifting is highly variable, the La Jencia Basin rates appear relatively high. However, the significant difference between peak and average La Jencia Basin rates modeled here highlights the potential for underestimation of many records of tectonic subsidence due to a lack of high-precision and high-resolution age constraints. Furthermore, age data and modeling results presented here document fault movement and consequent rates of tectonic subsidence that lower-resolution data would not, providing a high-fidelity case study of continental rift basin development.

New Mexico↗
USGS2026

Recycling radiogenic osmium by crustal foundering in subduction zones: Evidence from pyroxenite xenoliths in the north Andean arc

Removal of mafic-ultramafic lower crust (e.g., via delamination) is fundamental to making andesitic continental crust, yet direct evidence of this process remains elusive. A unique suite of garnet clinopyroxenite and hornblendite (arclogite) xenoliths from the northern volcanic zone of the Andes, erupted to the surface in the mid-Pleistocene, have bulk-rock geochemistry and osmium (Os) isotopic compositions that demonstrate their crustal affinity, yet have equilibration pressures and temperatures below the arc Mohorovičić discontinuity (~53 km) and some as deep as within the sub-arc mantle wedge (~105 km). Garnet websterites from the same xenolith suite, sourced from depths approaching the Wadati-Benioff zone (~140 km), have elemental and isotopic compositions indicative of a mantle origin, likely formed as products of peridotite-liquid reactions above the subducting slab. Variability in bulk-rock 143 Nd/ 144 Nd values and garnet oxygen isotope ratios ( 𝛿 18 O) for these samples is attributed to minor assimilation of subducted components and/or older crustal material, but assimilation alone cannot account for the highly radiogenic Os isotopic compositions. These results provide direct petrologic evidence for modern arclogite formation and foundering in the Andean orogen, the archetypal active continental subduction system. Elevated 187 Os/ 188 Os values of Mercaderes arclogites imply that recycling of gravitationally unstable arc cumulates during continental crust formation introduces highly radiogenic Os into the convective mantle, which has implications for mass fluxes across the crust-mantle boundary as well as the sources and evolution of mantle heterogeneity as seen in orogenic peridotite massifs and in the sources of oceanic basalts

Earth and Planetary Science Letters↗