Potassium‐argon ages on the Cainozoic volcanic rocks of New South Wales

During the Cainozoic there was widespread volcanism, mainly basaltic, in eastern New South Wales. Numerous new K‐Ar ages, together with previously published results, provide information on the age of virtually all the main volcanic provinces, and indicate that the volcanism started about 70 m.y. ago in the Late Cretaceous, and was continuous from about 60 m.y. ago (Palaeocene) until about 10 m.y. ago (middle Miocene). There has been no volcanic activity since 10 m.y. ago.

The ages of uplift of the Eastern Highlands are estimated from the relationship of the dated basaltic flows to the topography. A major uplift is deduced some time between the mid‐Cretaceous and late Oligocene, followed by a quiescent period. A further uplift started some time after the middle Miocene, and it continues to the present day. The highland was uplifted differentially both along and transverse to the axis.     

Metamorphic and metasomatic convergence of basic igneous rocks and lime‐magnesia sediments of the precambrian of North‐western Queensland∗

With increasing intensity of metamorphism, and particularly metasomatism, it becomes more and more difficult to separate amphibolites in North‐western Queensland into ortho‐ and para‐amphibolites, and a stage is reached at which the two are indistinguishable. Field evidence, carefully sought, has enabled the origin of some apparently identical rocks to be determined, and laboratory methods were applied to these rocks to see if grounds for the separation of amphibolites of unknown origin could be determined. None of the six methods tried — mineralogy (including feldspar twinning), chemical analysis (including TiO2 content), spectrography, rock magnetism — proved to be as successful as the field method in distinguishing the two rock groups; some methods used with apparent success elsewhere, such as feldspar twinning or chemical analyses, are found to be unsuccessful here, which suggests that though the problem is world‐wide, solutions are only of local validity.     

Rubidium‐strontium geochronology of the encounter bay granite and adjacent Metasedimentary rocks,South Australia

Rubidium‐strontium and strontium isotope data for eight whole‐rock samples of granite varieties from the Encounter Bay area, South Australia, yield an isochron age of 487 ± 37 m.y. Two specimens of albitised granite, formed as a result of late‐stage metasomatic alteration of original megacrystic granite, conform to this isochron. These data support a genetic relation between granites and late‐stage metasomatic alteration as suspected from field, petrographical and geochemical studies. Eight samples from contiguous Kanmantoo Group metasedimentary rocks have an isochron age of 487 ± 60 m.y. Thus this metamorphic event is coincident with emplacement of the Encounter Bay Granite.

The initial Sr⁸⁷Sr⁸⁶ ratio for the Encounter Bay Granite (0.719) is significantly higher than initial ratios for the Palmer (0.709) and Anabama (0.705) Granites from the same region and can be attributed to either remobilisation or incorporation of strontium from older crustal material in the intrusion. The apparent initial Sr⁸⁷/Sr⁸⁶ ratio for the Kanmantoo Group metasedimentary rocks (0.722) can not be distinguished from that for the Encounter Bay Granite within the analytical uncertainties. Compatability of ages and high initial Sr⁸⁷Sr⁸⁶ ratios suggest that the granites formed by remobilisation of associated crustal rock.     

Low‐K tholeiites and high‐K igneous rocks from Woodlark Island,Papua New Guinea

Woodlark Island, the largest above‐sea portion of the Woodlark Rise, has an exposed basement of pre‐Miocene (?Cretaceous‐Eocene) low‐K tholeiitic basalt and dolerite, and minor sediments. The basement is unconformably overlain by Early Miocene limestone and volcaniclastic sediments and later Miocene high‐K volcanics and comagmatic intrusives. Pleistocene to Recent sediments partly blanket the Tertiary sequence. Basement low‐K tholeiites vary only slightly in composition and are interpreted as ocean floor or possible marginal basin material. The high‐K suite appears to be chemically similar to late Tertiary to Recent high‐K igneous rocks of mainland Papua New Guinea. It includes porphyritic hornblende‐, clinopyroxene‐, biotite‐ and magnetite‐bearing shoshonite, latite and toscanite, and intrusive equivalents that range from olivine normative to strongly quartz normative compositions (S1Q2 46% to 75%). Computer mixing models indicate that separation of the pheno‐crysts in the shoshonites, particularly pargasitic hornblende, is a feasible mechanism for producing the more silica‐rich monzonites and latites.

The low‐K tholeiitic basement rocks of Woodlark Island are inferred to be part of an ophiolitic slab en echelon with the Papuan Ultramafic Belt, thrust over equivalents of the Cretaceous Owen Stanley Metamorphics or, in part, onto existing oceanic crust. High‐K igneous rocks on Woodlark Island appear to form an eastward extension of a province of calcalkaline to shoshonitic volcanic and intrusive rocks, which stretches from Mount Lamington to the Louisiade Archipelago. Late‐middle Miocene high‐K magmatism at Woodlark Island is consistent with the observation that activity commenced earlier in the E and became progressively younger westwards towards mainland Papua New Guinea. Periodicity in the magmatism was apparently synchronous with major rifting episodes that formed the Woodlark Basin. The data on the Woodlark Island high‐K suite support the currently accepted. concept of delayed partial melting of a mantle source previously modified by the introduction of water and LILE from an earlier subduction zone (Johnson et al., 1978b).     

Palaeomagnetic and potassium‐argon dating studies of the Tasmanian Dolerites

The stable magnetizations of the Tasmanian Dolerites are shown to fall into two distinct groups depending upon their directions and the geographical region of the dolerites. It has been suggested that this could be a result of significant age differences between the dolerites of these groups. A series of K‐Ar determinations indicates that there is no detectable systematic age differences and the average of the five bodies dated is 170.5 ± 8.0 m.y. (not significantly different from previous K‐Ar dates from a single body). A re‐appraisal of the palaeomagnetic data, in the light of the distinct groupings of the directions, yields two significantly different pole positions‐ Lat 50.7°S, Long. 174.5°E (A_9r, = 5.2°) and Lat. 47.7 °S, Long. 123.5° (A₉₅ = 9.5°)’. The former of these is in excellent agreement with pole positions from other Lower to Middle Jurassic rocks of Australia but the significance of the latter remains obscure.     

Response of U‐Pb Zircon and Rb‐Sr total‐rock and mineral systems to low‐grade regional metamorphism in proterozoic igneous rocks,mount Isa,Australia

A detailed Rb‐Sr total‐rock and mineral and U‐Pb zircon study has been made on suites of Proterozoic silicic volcanic rocks and granitic intrusions, from near Mt Isa, northwest Queensland. Stratigraphically consistent U‐Pb zircon ages within the basement igneous succession show that the oldest recognized crustal development was the outpouring of acid volcanics (Leichhardt Metamorphics) 1865 ± 3 m.y. ago, which are intruded by coeval, epizonal granites and granodiorites (Kalkadoon Granite) whose pooled U‐Pb age is 1862 ⁺²⁷ _‐21 m.y. A younger rhyolitic suite (Argylla Formation) within the basement succession has an age of 1777 ± 7 m.y., and a third acid volcanic unit (Carters Bore Rhyolite), much higher again in the sequence, crystallized 1678 ± 1 m.y. ago.

All of these rocks are altered in various degrees by low‐grade metamorphic events, and in at least one area, these events were accompanied by, and can be partly related to, emplacement of a syntectonic, foliated granitic batholith (Wonga Granite) between 1670 and 1625 m.y. ago. Rocks that significantly predate this earliest recognized metamorphism, have had their primary Rb‐Sr total‐rock systematics profoundly disturbed, as evidenced by 10 to 15% lowering of most Rb‐Sr isochron ages, and a general grouping of many of the lowered ages (some of which are in conflict with unequivocal geological relationships) within the 1600–1700 m.y. interval. Such isochrons possess anomalously high initial ⁸⁷Sr/⁸⁶Sr ratios, and some have a slightly curved array of isotopic data points. Disturbance of the Rb‐Sr total‐rock ages is attributed primarily to mild hydrothermal leaching, which resulted in the loss of Sr (relatively enriched in ⁸⁷Sr in the Sr‐poor (high Rb/Sr) rocks as compared with the Sr‐rich rocks).     

Geochemical characteristics of Cenozoic high-K igneous rocks from Liuhe-Xiangduo area, eastern Tibet

The major elements, trace elements and Nd-Sr isotopic composition of Cenozoic high-K igneous rocks and mafic deep-derived enclaves from the Liuhe-Xiangduo area, eastern Tibet, indicate the high-K igneous rocks are characterized as being enriched in Ca (CaO= 1.20% - 8.80% ), alkali (Na2O K2O= 3.47% - 10.65% ), especially K (K2O up to 5.96% ) and depleted in Ti (TiO2= 0.27% - 1.50% ). Their REE contents are very high (REE= 91.29 - 231.11 μg/g). Their REE distribution patterns are of the right-inclined type, characterized by intense LREE enrichment [(La/Yb)N= 7.44 - 15.73 ]. The rocks are distinctly enriched in Rb, Sr and Ba ( 46.3 -316 μg/g, 349-1220 μg/g and 386-2394 μg/g, respectively), high in U and Th ( 1.17 - 8.10 μg/g and 2.58 - 27.0 μg/g, respectively), moderate in Zr and Hf ( 87.5 -241 μg/g and 2.83 - 7.52 μg/g, respectively), and depleted in Nb and Ta ( 4.81 - 16.8 μg/g and 0.332 - 1.04 μg/g, respectively). In the primitive mantle-normalized incompatible element spidergram, U, K, Sr and Hf show positive anomalies, whereas Th, Nb, Ta, P, and Ti show negative anomalies. The rocks are strongly depleted in Cr and Ni ( 21.4 -1470 μg/g and 7.79 -562 μg/g, respectively), and their transition element distribution curves are obviously of type-W. The ( 87 Sr/ 86 Sr)i ratios range from 0.704184 to 0.707539 ; ( 143 Nd / 144 Nd)i from 0.512265 to 0.512564 ; and ε Nd (t) from -6.3 to -0.4 . These geochemical features might suggest that the potential source of the high-K igneous rocks in the Liuhe-Xiangduo area is similar to the EM2, which may be similar to the material enriched K that is located under the crust-mantle mixed layer. The mafic deep-derived enclaves in the high-K igneous rocks belong to chance xenoliths. Their ( 87 Sr/ 86 Sr)i ratios range from 0.706314 to 0.707198 ; ( 143 Nd / 144 Nd)i from 0.512947 to 0.513046 ; and ε Nd (t) from 7.0 to 9.0 . These geochemical features might indicate that the enclaves probably came from the depleted mantle. The P-T conditions of the enclaves also showed that the enclaves are middle-lower crust metamorphic rocks, which were accidentally captured at 20-50 km level by rapidly entrained high-K magma, whose source is over 50 km in depth.     

Geochemical characteristics of igneous rocks associated with epithermal mineral deposits—A review

Newly synthesized data indicate that the geochemistry of igneous rocks associated with epithermal mineral deposits varies extensively and continuously from subalkaline basaltic to rhyolitic compositions. Trace element and isotopic data for these rocks are consistent with subduction-related magmatism and suggest that the primary source magmas were generated by partial melting of the mantle-wedge above subducting oceanic slabs. Broad geochemical and petrographic diversity of individual igneous rock units associated with epithermal deposits indicate that the associated magmas evolved by open-system processes. Following migration to shallow crustal reservoirs, these magmas evolved by assimilation, recharge, and partial homogenization; these processes contribute to arc magmatism worldwide.Although epithermal deposits with the largest Au and Ag production are associated with felsic to intermediate composition igneous rocks, demonstrable relationships between magmas having any particular composition and epithermal deposit genesis are completely absent because the composition of igneous rock units associated with epithermal deposits ranges from basalt to rhyolite. Consequently, igneous rock compositions do not constitute effective exploration criteria with respect to identification of terranes prospective for epithermal deposit formation. However, the close spatial and temporal association of igneous rocks and epithermal deposits does suggest a mutual genetic relationship. Igneous systems likely contribute heat and some of the fluids and metals involved in epithermal deposit formation. Accordingly, deposit formation requires optimization of source metal contents, appropriate fluid compositions and characteristics, structural features conducive to hydrothermal fluid flow and confinement, and receptive host rocks, but not magmas with special compositional characteristics.     

Rb‐Sr systematics of the Claret Creek Ring Complex and their bearing on the origin of upper Palaeozoic igneous rocks in northeast Queensland

The Claret Creek Ring Complex forms a minor part of the extensive Upper Palaeozoic calcalkaline province of northeast Queensland. Although the Claret Creek Ring Complex contains 10 mappable units, it was formed about 300 m.y. ago over a time interval no greater than 10 m.y. This interval is short compared with the overall duration of Upper Palaeozoic igneous activity, which lasted from approximately 320 m.y. to 270 m.y. in this area. Although geochemically distinct, the complex shares a common initial ⁸⁷Sr/⁸⁶Sr ratio with the majority of the surrounding igneous rocks; this suggests derivation from different sources with a common initial ratio. Such a relationship could arise by the re‐melting or pronounced fractional crystallisation of magmas which underplated or intruded the lower crust immediately prior to final magma generation. Alternatively, the acidic magmas may have originally formed by partial melting of crustal rocks immediately after a regional isotopic homogenisation. In either case the magmas were derived from originally igneous rocks which were dioritic in chemical composition.     

Age and strontium‐isotope geochemistry of differentiated rocks from the newer Volcanics,MT Macedon Area,Victoria, Australia

The Newer Volcanics Province of Victoria and South Australia consists of a major region of mainly alkaline basalts within which are two restricted areas containing strongly differentiated flow‐rocks. Typical alkalic basalts from this widespread province have K‐Ar ages from 4.5 to 0.5 m.y. and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7038 to 0.7045. Contrastingly, in the Macedon area of differentiated lavas, flow compositions range from limburgite to soda trachyte, with K‐Ar ages from 6.8 to 4.6 m.y. and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7052 to 0.7127. These differentiated rocks therefore are older, and some of them may have been contaminated by reaction with more radiogenic basement rocks during differentiation. Alternatively, the variation in initial Sr‐isotope composition may have resulted from varying isotopic composition of partial melts from the immediate source rocks. The most felsic of the differentiated rocks, soda trachyte, is extremely enriched with Rb relative to Sr; one of the three restricted outcrops of this rock (Camel's Hump) yields a total‐rock Rb‐Sr isochron age of 6.3 ± 0.6 m.y. with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7127. K‐Ar sanidine ages reported for the three outcrops of trachyte are identical to each other and to the Rb‐Sr isochron result.     

The age of orogenesis in the Nambucca Slate Belt: A K‐Ar study of low‐grade regional metamorphic rocks

K‐Ar ages of biotite and hornblende from undeformed granodiorite plutons and of slaty and phyllitic rocks, ranging from prehnite‐pumpellyite metagreywacke to greenschist fades, have been determined in an attempt to define the age of orogenesis in the eastern part of the Nambucca Slate Belt. The plutons have K‐Ar ages of 226–227 m.y. (biotite) and 228–231 m.y. (hornblende) that provide a younger age limit for deformation. The lower grade metamorphic rocks yield a range of ages including some comparable with the depositional age of the rocks as indicated by fossils. Rocks of pumpellyite‐actinolite and greenschist facies give a more coherent group of ages which suggest orogenesis at about 250–255 m.y. Specimens of these latter rocks that have been affected by a later structural episode than that during which slaty cleavage formed, yield slightly older ages, which may result from the inclusion of minor amounts of environmental excess ⁴⁰Ar.

Support for the 250–255 m.y. age comes from previously determined radiometric ages from the western part of the Slate Belt, although the presence of granitic bodies perhaps as old as 289 m.y., some closely associated with high‐grade regional metamorphic rocks, may indicate the presence of additional earlier orogenic movements in this region.     

The structure of a small complexly‐deformed area within Ordovician rocks near Mansfield,Victoria

Two distinct groups of granitoids occur on the eastern side of the Kosciusko Batholith. Those considered to be derivatives of sedimentary source rocks (S‐types) are usually foliated and either contain cordierite or white‐mica secondary after cordierite. The granitoids produced from igneous source material (I‐types) are generally massive and frequently contain hornblende. Geochemical parameters provide the best discriminant between the two groups, I‐types have higher Ca, Al, Na₂O/K₂O, and Fe₂O₃/FeO, and lower Fe, Mg, Sc, V, Cr, Co, Ni, Cu, Zn, Ba, Rb, Th, La, Ce, and Y than S‐types of comparable SiO₂ values.

The differences between the two groups are not the result of differences in the melt‐forming process but reflect differences in the nature of the source material. Thus the geochemical features of the S‐type granitoids are indicative of their source rocks having been through a process of chemical weathering in a sedimentary cycle. Conversely, the I‐type granitoids were derived from fractionated rocks that had not been involved in weathering processes.     

K‐Ar ages of Cainozoic volcanic suites,Bowen‐St Lawrence Hinterland,North Queensland (with some implications for petrologic models)

Thirty K‐Ar dates on Cainozoic volcanic rocks lying at the north end of the Bowen Basin suggest that several episodes of volcanism took place at major structural weaknesses. The oldest volcanism (ca 54 m.y.) was located outside the basin structure. The main volcanism (Nebo and East Clermont Provinces) extended from early Oligocene (34–35 m.y.) to mid‐Cainozoic time (21–22 m.y.?). Isolated Pliocene activity is tentatively suggested by dates on Mt St Martin (ca 3 m.y.).

Dating of the Nebo central volcano (31–33 m.y.) supports the model of Wellman &; McDougall, with volcanic activity related to migration of Australia northwards over a mantle magma source. Consideration of the Nebo dates with those of other central volcanoes in north Queensland, suggests that central felsic activity was surrounded by broad zones of peripheral eruptives, petrologically zoned from outer undersaturated basalts to inner saturated basalts. These zones (super provinces) delineate the size and profile of underlying magma sources and appear to trend back in time and space to sea‐floor spreading episodes in the Coral Sea—southeastern Papua region (55 m.y.).

The basalt dates also assist in fixing periods of lateritization (mid‐Oligocene) and in determining approximate minimum erosion rates in the northern Bowen Basin since the Eocene (3–5m/m.y.).     

Potassium‐argon ages on micas from the Precambrian region of North‐Western Queensland

Potassium‐argon measurements have been carried out on the separated micas of 27 samples, principally granites, from the Mount Isa‐Cloncurry region of north‐western Queensland. There is evidence for at least two tectonic periods within the “Lower Proterozoic” of the area. The first is represented only in the north‐western portion, with ages greater than 1,770 m.y. on the Ewen Granite, and on the granites of the Nicholson River area to the far north‐west. The second at 1,400–1,450 m.y. is manifested only to the south and east of the Kalkadoon‐Leichhardt complex, and including the Sybella Granite. The results may be further interpreted as lending support to the concept of a possible “metamorphic discontinuity” along the western flank of the Kalkadoon‐Leichhardt complex, postulated by Carter, Brooks and Walker; as suggesting possible contemporaneity of the Cliffdale Volcanics and the Argylla Formation; and as giving further evidence for the antiquity of stromatolites. Comparison with earlier work suggests that some deposits in this region may be contemporaneous with some of the Agicondian sediments of the Katherine‐Darwin area, and that the second tectonic period corresponds with the K‐Ar ages obtained on the Davenportian granites of Central Australia.     

Emplacement ages and sources of kimberlites and related rocks in southern Africa: U–Pb ages and Sr–Nd isotopes of groundmass perovskite

Groundmass perovskite has been dated by LA-ICPMS in 135 kimberlites and related rocks from 110 localities across southern Africa. Sr and/or Nd isotopes have been analysed by LA-MC-ICPMS in a subset of these and integrated with published data. The age distribution shows peaks at 1,600–1,800, 1,000–1,200, 500–800 and 50–130 Ma. The major “bloom” of Group I kimberlites at ca 90 ± 10 Ma was preceded by a slow build-up in magmatic activity from ca 180 Ma. The main pulse of Group II kimberlites at 120–130 Ma was a distinct episode within this build-up. Comparison of the isotopic data with seismic tomography images suggests that metasomatized subcontinental lithospheric mantle (SCLM) with very low ε _Nd and high ⁸⁷Sr/⁸⁶Sr, (the isotopic signature of Group II kimberlites) was focused in low-Vs zones along translithospheric structures. Such metasomatized zones existed as early as 1,800 Ma, but were only sporadically tapped until the magmatic build-up began at ca 180 Ma, and contributed little to the kimberlitic magmas after ca 110 Ma. We suggest that these metasomatized volumes resided in the deep SCLM and that their low-melting point components were “burned off” by rising temperatures, presumably during an asthenospheric upwelling that led to SCLM thinning and a rise in the ambient geotherm between 120 and 90 Ma. The younger Group I kimberlites therefore rarely interacted with such SCLM, but had improved access to shallower volumes of differently metasomatized, ancient SCLM with low ⁸⁷Sr/⁸⁶Sr and intermediate ε _Nd (0–5). The kimberlite compositions therefore reflect the evolution of the SCLM of southern Africa, with metasomatic-enrichment events from as early as 1.8 Ga, through a major thermal and compositional change at ca 110 Ma, and the major kimberlite “bloom” around 90 Ma.     

The petrology of the Mt Manypeaks Adamellite and associated high‐grade metamorphic rocks near Albany,Western Australia

The Mt Manypeaks Adamellite is a composite, regionally concordant pluton at least 22 km long and 3 km wide, associated with Precambrian amphibolite facies gneisses of the Albany‐Esperance Block, and situated about 35 km east of Albany, Western Australia. The pluton is surrounded by a granitised aureole, and shows structural and mineralogical harmony with the country rocks. Contacts vary from grada‐tional to sharp. Hence field relations are consistent with syn‐ or late‐kinematic emplacement in the catazone. The normative composition of the pluton corresponds with the thermal trough in the system An‐Ab‐Or‐Q‐H2O at 7 kb PH2O, suggesting an origin involving crystal‐melt equilibria. The pluton is believed to have formed almost in situ by partial anatexis of the country rocks at 700–750°C and a depth of about 25 km during the orogenic episode responsible for regional metamorphism and deformation.     

From Gondwana to Europe: The journey of Elba Island (Italy) as recorded by U–Pb detrital zircon ages of Paleozoic metasedimentary rocks

Elba Island, located midway between Corsica and mainland Italy, is a small but important fragment of the Adria Plate. It has a rich sedimentary record preserved in a stack of tectonic nappes of both continental margin and oceanic origin. Especially the detrital zircons in early Paleozoic to early Mesozoic metasedimentary rocks provide an archive of many important geological events in the island's history. Elba Island and Adria originated along the northern margin of Gondwana, but drifted north in Silurian times to become part of Europe. A large new dataset of LA-ICP-MS and SIMS U–Pb zircon ages allows us to trace this history. Three main stratigraphic units have been investigated. The oldest Porto Azzurro Unit was deposited in the early Cambrian and has zircon age distributions indicating a typical northern African provenance, most likely sourced from the Saharan Metacraton. The Ortano Unit has a simple, mostly unimodal Ordovician age distribution that is entirely dominated by metavolcanic rocks and their erosional products; a sample of the metavolcanic Ortano Porphyroids provided a SIMS U–Pb zircon age of 460 ± 3 Ma. This phase of intense volcanism is related to the subduction of the Rheic Ocean beneath Gondwana, terminating with initial rifting and subsequent opening of the Paleotethys. This also marks the onset of the separation of a range of European terranes, including Adria and future Elba Island, from Gondwana. The Permo-Triassic Monticiano–Roccastrada Unit is the first to show a European provenance with the appearance of large amounts of Variscan and late to post-Variscan detritus. The presence of Variscan detrital zircons in the Permo-Triassic sediments is unexpected, since a Variscan age signature is so far not well recorded in the Adria Plate. This dataset is the most comprehensive detrital zircon dataset so far available for the Adria Plate and documents Adria's close affinity to Africa in the Lower Paleozoic, as well as its initial rifting within an active continental margin setting during the Ordovician and its final separation and independent evolution since late Palaeozoic times.     

Late Palaeozoic igneous rocks of the Great Xing’an Range,NE China: the Tayuan example

ABSTRACT

The Tayuan plutons located at the boundary of the Erguna and Xing’an blocks expose a coexisting mafic–felsic association that is made of monzogranite and gabbro-monzodiorite as well as subordinate quartz monzonite. LA–ICP–MS U–Pb zircon dating revealed a synchronous emplacement of the monzogranite (314–317 Ma), gabbro (308–315 Ma), and quartz monzonite (310 ± 3 Ma). The majority of these intrusions are characterized by an enrichment in light rare earth elements relative to heavy rare earth elements and a depletion of high strength field elements (e.g. Nb, Ta, Ti). Zircons from the gabbro and monzogranite have ε_Hf(t) values of 1.1–9.6 and ?3.0–3.3, respectively. Geochemical data show that the gabbro-monzodiorite may have been generated by the melting of a fluid-metasomatized lithospheric mantle, while the monzogranite may have been formed by a partial melting of the Mesoproterozoic crust. The quartz monzonite has similar whole-rock geochemical and Hf isotopic compositions to those of the gabbros and could have been produced from the same mantle source as that from which the gabbros were extracted. The Tayuan plutonic rocks have high contents of K₂O and total alkalis and show a northwestward polarity like that of the continental margin plutonic rocks along the Hegenshan–Heihe suture zone. Combined with data from published studies, our data indicate that the Tayuan intrusive rocks were generated by the northwestward subduction of the Hegenshan–Heihe Oceanic plate.