Sr Isotope Initial Ratios Research Articles

Shrimp U–Pb zircon geochronology, geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province, SW China

The Cida A-type granitic stock (∼ 4 km 2) and Ailanghe I-type granite batholith (∼ 100 km 2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO ⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites. The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios ( ε Nd( t) = − 0.25 to + 1.35 and ( 87Sr/ 86Sr) i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε Nd( t) ratios (− 6.34 to − 6.26) and high ( 87Sr/ 86Sr) i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks. In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.

Compositional diversity of Eocene–Oligocene basaltic magmatism in the Eastern Rhodopes, SE Bulgaria: implications for genesis and tectonic setting

Basaltic magmatism occurred only rarely within the extensive Eocene–Oligocene volcanic activity in the Eastern Rhodope Mts., SE Bulgaria. The earliest mafic volcanism started at ca. 34 Ma with K-rich trachybasalts strongly enriched in large ion lithophile elements (LILE), particularly Ba, Sr, Pb, Th, and light rare earth elements (REE) relative to the high field strength elements (HFSE). They have high 87Sr/ 86Sr ratios (0.70688–0.70756), low 144Nd/ 144Nd (0.51252–0.51243), and very high 207Pb/ 204Pb (15.74–15.76) and 208Pb/ 204Pb (39.07–39.14) at low 206Pb/ 204Pb (18.72–18.73) ratios, reflecting high degrees of crustal contamination. Shoshonitic basalts and absarokites and calc-alkaline and high-K calc-alkaline magmas, which erupted between 33 and 31 Ma, have decreasing Sr isotope initial ratios from west (0.70825) to east (0.70647) at approximately constant 143Nd/ 144Nd isotopic compositions (0.51252–0.51243) and slightly decreasing 207Pb/ 204Pb (15.66–15.72) and 208Pb/ 204Pb (38.80–38.96) and increasing 206Pb/ 204Pb (18.73–18.90) in comparison with the trachybasalts. All these rocks are characterized by negative Nb–Ti and Eu anomalies. They resulted from different degrees of partial melting of enriched asthenosphere, and the magmas were later contaminated by the Rhodopian crust. The end of the magmatic activity (28–26 Ma) was marked by emplacement of alkaline dykes, spatially associated with metamorphic core complexes. They are characterized by low 87Sr/ 86Sr (0.70323–0.70338), high 144Nd/ 144Nd (0.51290–0.51289), and 206Pb/ 204Pb (18.91–19.02) at lower 207Pb/ 204Pb (15.52–15.64) and 208Pb/ 204Pb (38.59–38.87) ratios, consistent with an origin from a source similar to OIB-like European Asthenospheric reservoir contaminated by depleted mantle lithosphere. The Eastern Rhodope Eo-Oligocene mafic magmatism formed as part of the prolonged extensional tectonics of the whole Rhodope region in Late Cretaceous–Paleogene time, similar to those in the U.S. Cordillera and Menderes Massif (Turkey). Initiation of extension is constrained by the formation of metamorphic core complexes, low-angle detachment faults, and supradetachment Maastrichtian–Paleocene sedimentary basins, intimately associated with 70–42 Ma granitoids and metamorphism which record mantle perturbation. The Eo-Oligocene stage started with block faulting, sedimentary basin formation, and extensive acid-intermediate and basic volcanism over the entire Eastern Rhodope area. The order of emplacement of the basalts from high-Ba trachybasalts through shoshonites, calc-alkaline and high-K calc-alkaline basalts, and finally to purely asthenospheric-derived alkaline basalts, with progressively decreasing amount of crustal component, reflects upwelling asthenospheric mantle. Most of the models proposed in the literature to explain extension and magmagenesis in the Rhodopes and the Mediterranean region cannot be applied directly. Critical evaluation of these models suggest that some form of convective removal of the lithosphere and mantle diapirism provide the most satisfactory explanation for the Paleogene structural, metamorphic, and magmatic evolution of the Rhodopes.

The Anfeg post-collisional Pan-African high-K calc-alkaline batholith (Central Hoggar, Algeria), result of the LATEA microcontinent metacratonization

The Anfeg batholith (or composite laccolith) occupies a large surface (2000 km 2) at the northern tip of the Laouni terrane, just south of Tamanrasset in Hoggar. It is granodioritic to granitic in composition and comprises abundant enclaves that are either mafic microgranular enclaves (MME) or gneissic xenoliths. It intruded an Eburnian (≈2 Ga) high-grade basement belonging to the LATEA metacraton at approximately 608 Ma (recalculated from the U–Pb dating of [Tectonics 5 (1986) 955]) and cooled at approximately 4 kbar, with a temperature of about 750 °C. This emplacement occurred mainly along subhorizontal thrust planes related to Pan-African subvertical mega-shear zones close to the attachment zone of a strike-slip partitioned transpression system. Although affected by some LILE mobility, the Anfeg batholith can be ascribed to a high-K calc-alkaline suite but characterized by low heavy REE contents and high LREE/HREE ratios. The MME belong to the Anfeg magmatic trend while some xenoliths belong to Neoproterozoic island arc rocks. The Anfeg batholith defines a Nd–Sr isotopic initial ratios trend ( ε Nd/( 87Sr/ 86Sr) i from −2.8/0.7068 to −11.8/0.7111) pointing to a mixing between a depleted mantle and an old Rb-depleted granulitic lower crust. Both sources have been identified within LATEA and elsewhere in the Tuareg shield ( ε Nd/ 87Sr/ 86Sr) i of +6.2/0.7028 for the depleted mantle, −22/0.708 for the old lower crust. The model proposed relates the above geochemical features to a lithospheric delamination along the subvertical mega-shear zones that dissected the rigid LATEA former passive margin without major crustal thickening (metacratonization) during the general northward tectonic escape of the Tuareg terranes, a consequence of the collision with the West African craton. This delamination allowed the uprise of the asthenosphere. In turn, this induced the melting of the asthenosphere by adiabatic pressure release and of the old felsic and mafic lower crust due to the high heat flow. A gradient in the mantle/crust ratio within the source of the Pan-African magmatism is observed in LATEA from the northeast (Egéré-Aleksod terrane) where rare plutons are rooted within the Archaean/Eburnian basement to the southwest (Laouni terrane) where abundant batholiths, including Anfeg, have a mixed signature. Some mantle melts with only slight crustal contamination (Laouni troctolitic layered intrusions) are even present. This suggests that the southern boundary of LATEA microcontinent is not far south of the Tuareg shield.

Petrology of the Cenozoic volcanism in the Upper Benue valley, northern Cameroon (Central Africa)

Thirty-one plugs of alkaline volcanic rocks of Cenozoic age (37 Ma in mean) occur in the Upper Benue valley, northern Cameroon (Central Africa). The complete alkaline series (alkaline basalts, hawaiites, mugearites, phonolites, trachytes and rhyolites) is represented. Basalts contain phenocrysts of olivine, Al-Ti-rich diopside, and Ti-magnetite, and hawaiites-abundant microphenocrysts of plagioclase. Mugearites have a trachytic texture and contain xenocrysts of K-feldspar, apatite, quartz and unstable biotite. Phonolites are peralkaline. Trachytes (peralkaline and non-peralkaline) and rhyolites are characterised by their sodic mineralogy with aegirine-augite, richterite, and arfvedsonite phenocrysts. There is a large compositional gap between basaltic and felsic lavas, except the mugearites. Despite this gap, major- and trace-element distributions are in favour of a co-magmatic origin for the basaltic and felsic lavas. The Upper Benue valley basalts are similar in their chemical and isotopic features to other basalts from both the continental and oceanic sectors of the Cameroon Line. The Upper Benue valley basaltic magmas (87Sr/86Sr≈0.7035; e Nd=+3.9) originate from an infra-lithospheric reservoir. The Sr–Nd isotopic composition and high Sr contents of the mugearites suggest that they are related to mantle-derived magmas and that they result from the mixing, at shallow crustal levels, of a large fraction of trachytic magma with a minor amount of basaltic magma. Major-element modelling of the basalt–trachyte evolution (through hawaiite and mugearite compositions) does not support an evolution through fractional crystallization alone. The fluids have played a significant role in the felsic lavas genesis, as attested by the occurrence of F-rich minerals, calcite and analcite. An origin of the Upper Benue valley rhyolitic magmas by fractional crystallization of mantle-derived primitive magmas of basaltic composition, promoted or accompanied by volatile, halogen-rich fluid phases, may be the best hypothesis for the genesis of these lavas. These fluids also interact with the continental crust, resulting in the high Sr-isotope initial ratios (0.710) in the rhyolites, whereas the Nd isotopic composition has been less affected (e Nd=+0.4).

Mafic Cumulus Processes and Chemical Evolution in an Open Magma Chamber: The Newark Island Layered Intrusion, Labrador, Canada

The Newark Island Layered Intrusion (NILI) is a Middle Proterozoic (1305 ± 5 Ma; U-Pb zircon) mafic pluton on the eastern edge of the Nain Anorthosite Complex, Labrador. The NILI has been subdivided by Wiebe (1988) into a stratigraphically lower, earlier, layered series (LS) and a later hybrid series (HS). Although grossly similar in mineralogy, the REE and Sr isotope systematics of the LS and HS cumulates differ dramatically. All but one of the LS cumulates exhibit low total REE (La = 5 to 8 X chondrites) and a large positive Eu anomaly, indicative of an adcumulus to mesocumulus origin, with plagioclase and olivine as the expected major cumulus phases. Analyzed HS cumulates and other nonchilled rocks also contain principally olivine and plagioclase as cumulus phases, yet exhibit much higher abundances of the REE (La = 40 to 90 X chondrites), are LREE enriched ((Ce/Sm)n = 1.78), and show only a small positive Eu anomaly that diminishes with increasing REE abundances. No assemblage of mafic minerals alone could have generated these patterns and abundances. This suggests that HS mafic cumulates must contain a substantial amount of trapped liquid, and therefore are classified as orthocumulates. Modeling of the compatible element-incompatible element ratio Sr/Ba confirms the need for a substantial trapped liquid component in HS nonchilled mafic rocks. Sr isotopic studies indicate a restricted range for the HS cumulates (87Sr/86Srinitial = 0.7036 to 0.7040), which is indistinguishable from chilled mafic pillows (= liquids) found from within the HS. LS cumulates exhibit an extremely broad range (87Sr/86Srinitial = 0.7050 to 0.7081), which is elevated relative to any mafic rock found in the HS. The relatively elevated and variable Sr isotopic initial ratios for LS cumulates may indicate selective contamination with a more radiogenic component (crust?) in the early stages of NILI magmatism. With the intrusion of HS magmas, magmatism reached its xenith and contamination was no longer a factor. Early LS magmatism may have “armored” the conduit(s) that fed the NILI chamber and allowed later magma pulses, such as those that gave rise to the HS, to arrive at the level of the intrusion relatively uncontaminated. This scenario is analogous to what has been hypothesized for successive volcanic eruptions in the Aleutian Islands, Alaska (e.g., Myers et al., 1985; Meyers and Frost, 1994).

Magma replenishment, and the significance of poikilitic texlures, in the Lower Main Zone of the western Bushveld Complex, South Africa

AbstractPetrographic and compositional variations in the Lower Main Zone (LMZ) of the western Bushveld Complex indicate changing regimes of magma replenishment. The lowermost unit of the LMZ, designated N-I, is an enigmatic sequence of leuconoritic cumulates, characterized primarily by up-sequence increases in both orthopyroxene Mg# and whole-rock Sr isotope initial ratio. The Sr isotope profile of N-I is ascribed to injection and progressive integration of small influxes of fresh magma with high (Main Zone-type) Sr isotope initial ratios. The basal Fe-enrichment in N-I, on the other hand, is ascribed to a separate, later mechanism involving the downward migration of late-stage Fe-rich liquids. The overlying two units, N-II and G-I, delineated chiefly in terms of basal Mg-enrichment of orthopyroxene, are ascribed to injections of fresh magma into the chamber. Poikilitic orthopyroxene grains in the basal parts of both N-II and G-I suggest entrainment and partial resorption of plagioclase grains from the semi-crystalline resident material into which the fresh magma was intruded.

Geochemistry of granitic rocks from the Hercynian Sardinia-Corsica batholith: Implication for magma genesis

The Sardinia-Corsica batholith was structured in the late stage of the Hercynian orogenesis. The granitoids intrude mainly metamorphic complexes grading from zeolite up to amphibolite facies. The batholith is heterogeneous consisting of complexes with different affinity, chemical composition, age and degree of deformation. The present paper reports major-and trace-element data for selected samples coming only from Sardinian outcrops. The rocks range from gabbro-diorite to tonalite, monzogranite and leucogranite. The two latter lithologies are the most abundant, gabbrodiorites and tonalites occurring in minor amounts and mainly in northern-central Sardinia. Over 75% of the granitoids contain microgranular enclaves of magmatic origin. The age of the rocks falls in the interval between 307 and 281 Ma. Sr isotope initial ratios are high, ranging between 0.7083 and 0.7107. REE, Rb, Sr, Ba, Zr, Th, Ta, Hf, Co and Sc abundances were determined on selected samples. All elements follow three types of trends vs. CaO, which is used as differentiation index. Two trends show positive and negative correlations while the third one shows a bell-shaped pattern. LREE have different degrees of enrichment (La = 20−120× ch) and HREE show variable fractionation with prevailing ( Tb Yb ) n <1 . The two peraluminous samples have very different geochemical characteristics. From the geochemical point of view all the rocks coming from the Sardinian segment of the batholith display a typical calc-alkaline chemical character showing the imprint of both “normal and mature” continental arc geodynamic environments. Geochemical trends suggest some petrogenetic constraints. The complete sequence of differentiation can be neither the product of crystal/liquid fractionation processes starting from a single basic parent magma nor the product of an AFC process. On the contrary, a two-stage model can be proposed. In the first stage a mafic melt of subcrustal origin interacted with monzogranitic magmas derived from 25–35% degree of melting of a crustal biotite amphibolitic source. Such a mixing process acted together with a crystal/liquid fractionation process to give tonalites and granodiorites. In the second stage lesser degrees of melting of the same crustal source could give the late-stage leucogranitic masses. A possible scenario, able to take into account field and geochemical data, can be suggested for the genesis of this suite and we propose it as a working model for future investigations.

Relationships between crustal contamination and crystallisation in continental flood basalt magmas with special reference to the Deccan Traps of the Western Ghats, India

The upper part of the Deccan Traps sequence (Bushe to Mahabaleshwar Formations) shows a statistically significant tendency for the most mafic lavas to be the most contaminated by crustal materials. This is the reverse of the relationship shown by suites evolving by contamination accompanied by fractional crystallisation (AFC). The observed correlations (e.g. between Mg-number and Sr isotope initial ratios) are partly an accidental consequence of the fact that the most mafic lavas are more abundant in the lower part of the sequence, while contaminant availability declines in the upper part. It is probable, however, that the correlations are augmented by increased contamination of hotter magma batches during ascent through dykes, a process during which fractional crystallisation is suppressed by magmatic turbulence. The absence of AFC relationships suggests that most of the contamination took place during the ascent stage rather than in a magma chamber. Other continental flood basalt provinces such as the Parana and Etendeka do show AFC relationships, and it is speculated that this may be a result of magma chamber contamination coupled with flow rates which prevent contamination during ascent.

Genesis of acid/basic rock associations: a case study The Kallithea intrusive complex, Samos, Greece

The Kallithea intrusive complex on Samos forms part of the Miocene granitoid province of the central Aegean. The complex consists of numerous composite dikes consisting of different I-type diorites, monzodiorites, (quartz) monzonites, granodiorites, and granites, as well as rare pegmatites. Within individual dikes the different rock types display various structural relationships to each other, most of which indicate that multiple intrusion was the main process responsible for the association of different rock types. Petrographical, geochemical, and Sr isotope data prove that at least some of the different magma pulses were genetically unrelated. For others, a comagmatic relationship cannot be excluded. The most spectacular feature of the composite dikes are net-veined parts in which spherical (pillow-like) to angular bodies of microdiorite are surrounded by a network of more felsic rocks of varying compositions (monzonites, granodiorites, and monzogranites). — For the microdiorite/monzogranite pairs, a formation by unmixing due to liquid immiscibility is suggested by the following facts: (a) the presence of monzogranite ocelli within the microdiorite bodies, (b) similar compositions of those minerals present in both the basic and felsic parts, (c) the enrichment of HFS elements in the basic parts and the depletion of these elements in the acid parts, (d) similar Sr isotope initial ratios. Such an origin, however, is excluded for the other net-veined parts having felsic veins of either monzonitic or granodioritic compositions. In these pairs, the HFS elements are enriched in the acid parts, common minerals may have different compositions, and Sr isotope initial ratios are different. These net-veined parts can only be explained by the model of multiple injections whereby a felsic melt intruded into a basic magma.

Petrogenesis of monzonoritic dykes in the Egersund-Ogna anorthosite (Rogaland, S.W. Norway): trace elements and isotopic (Sr, Pb) constraints

Two major monzonoritic dykes occur in the Egersund-Ogna anorthositic massif (S.W. Norway): the Lomland dyke, which varies from norite to monzonite, and the Vettaland ferronoritic dyke. They are characterized by high Fe, Ti and P contents, low SiO2, variable K2O (0.5%–4.5%) and high Fe/Mg ratios. Small variations in REE distribution are observed inside the Lomland dyke [La/Yb=12; LREE ca. 150–180 (chondrite-normalized values), neutral to slightly positive Eu anomaly]. Part of the Vettaland dyke is severely depleted in Zr, Rb and REE and shows a positive Eu anomaly. All rocks are depleted in U and Th, and show very low Cr and Ni contents, as well as high Sr contents (400–600 ppm). Variation within the Lomland dyke is satisfactorily ex-plained through subtraction of an apatite-bearing noritic cumulate. The role of apatite is predominant in controlling the REE behaviour. Eutectic partial melting of a Fe-rich noritic cumulate (containing apatite) under low pH2O conditions is suggested for the Vettaland dyke by REE modelling. Sr isotopic initial ratios are different in Lomland (ca. 0.708) and Vettaland (ca. 0.706) dykes. These values preclude direct derivation from the mantle as well as comagmatic relationship with anorthosite. Contamination of a mantle derived magma by deep crustal material or anatexis of the LIL depleted lower crust is compatible with the low U and Th contents, and with the Pb isotopic ratios. The Vettaland partial melting process is extended to all monzonoritic types, the necessary heat being provided by the high temperature anorthositic crystal mush.

Chemical heterogeneity of the Archaean mantle, composition of the earth and mantle evolution

New rare earth element (REE) data for Archaean basalts and spinifex-textured peridotites (STP) show a range of La/Sm ratios (chondrite-normalized) from 0.36 to 3.5, with the bulk of the data in the range 0.7–1.3. This supports the hypothesis, based on Sr isotope initial ratios, that the Archaean mantle was chemically heterogeneous. We suggest that the bulk mantle source for Archaean basaltic magmas was close to an undepleted earth material. An average chemical composition of the Archaean mantle is estimated using chemical regularities observed in Archaean STP and high-magnesian basalts. TiO 2 and MgO data show an inverse correlation which intersects the MgO axis at about 50% MgO (Fo 92). TiO 2 abundance in the mantle source is measured on this plot by assigning an MgO= 38% for the mantle. Concentrations of other elements are also estimated and these data are then used to obtain a composition for the bulk earth. We suggest an earth model with about 1.35 times ordinary chondrite abundances of refractory lithophile elements and about 0.2 times carbonaceous type 1 chondrite abundances of moderately volatile elements (such as Na, Rb, K, Mn). P shows severe depletion in the model earth relative to carbonaceous chondrites, a feature either due to volatilization or core formation (preferred). Our data support the hypothesis of Ringwood that the source material for the earth is a carbonaceous chondrite-like material. The generation of mid-ocean ridge basalts (MORB) is examined in the light of the model earth composition and Al 2O 3/TiO 2, CaO/TiO 2 ratios. It is suggested that for primitive basalts, these values can be used to predict the residual phases in their source. Comparison of chemical characteristics of inferred sources for 2.7-b.y. Archaean basalts and modern “normal” MORB indicates that the MORB source is severely depleted in highly incompatible elements such as Cs, Ba, Rb, U, Th, K, La and Nb, but has comparable abundances of less incompatible elements such as Ti, Zr, Y, Yb. The cause of the depletion in the MORB source is examined in terms of crust formation and extraction of silica-undersaturated melts. The latter seems to be a more likely explanation, since the degree of enrichment of highly incompatible elements in the crust only accounts for up to 40% of their abundances in the bulk earth and cannot match the depletion pattern in normal MORB. A large volume of material, less depleted than the source for normal MORB must therefore exist in the mantle and can serve as the source for the ocean island basalts and “normal” MORB. Three different mantle evolution models are examined and each suggests that the mantle is stratified with respect to abundances of incompatible trace elements. We suggest that no satisfactory model is available to fully explain the spectrum of geochemical and geophysical data. In particular the Pb and Sr isotope data on oceanic basalts, the depletion patterns of MORB and the balance between lithophile abundances in the crust and mantle, are important geochemical constraints to mantle models. Further modelling of the mantle evolution will be dependent on firmer information on the role of subduction, mantle convection pattern, and basalt production through geologic time together with a better understanding of the nature of Archaean crustal genesis.

Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho

Mesozoic orogeny and magmatism began in the northwestern United States soon after deposition of Permian strata, but no rocks have yet been dated from the Permian-Triassic orogenic period. Middle Triassic to Late Jurassic sediment sequences include major unconformities and evidence of several episodes of igneous activity. An early culmination of magmatism occurred in Late Triassic and Early Jurassic time (200–217 m.y. ago) in eugeosynclinal parts of far western Idaho. A widespread and intense culmination in Late Jurassic time was the final major orogenic event in the Oregon eugeosyncline. The Bald Mountain (147 to 158 m.y. old), Wallowa (probably 143 to 160 m.y. old but affected by Cretaceous metamorphism), Deep Creek (at least 137 m.y. old), and many other plutons in the Blue and Klamath Mountain regions in Oregon and in western Idaho were emplaced shortly before the end of Jurassic. The bulk of the Idaho batholith was emplaced during a Cretaceous culmination of igneous activity — the southern (Atlanta) lobe about 75 to 100 m.y. ago and the northern (Bitterroot) lobe about 70 to 80 m.y. ago. Much of the batholith was affected by Eocene magmatism which resulted in widespread resetting of isotopic dates for older rocks to values of 50 m.y. or less. Between 55 and 70 m.y. ago, there was a lull in igneous activity in the northwestern United States. Sr isotope initial ratios change abruptly across a boundary in western Idaho from ∼0.7040 or less, to the west, to ∼0.7060 or greater, to the east. This change marks the boundary between Precambrian crust and Phanerozoic eugeosyncline. The geologic setting of the observed transition and its time independence suggest that it is due to contamination and assimilation processes involving magmas from the mantle and enclosing crustal rocks. Contamination of magmas with radiogenic Sr renders the Sr whole-rock isochron technique useless in dating the Idaho batholith and other intrusive rocks in central and eastern Idaho, areas underlain by Precambrian basement.