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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Diamond-destructive mantle metasomatism: Evidence from the internal and external textures of diamonds and their nitrogen defects

    Y. FedortchoukI.L. ChirmS.H. Perritt
    19页
    查看更多>>摘要：Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kirnberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic li-thology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds. Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions.
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    Oxide enrichment by syntectonic melt-rock interaction

    Hindol GhatakRobyn L. GardnerNathan R. Daczko
    16页
    查看更多>>摘要：Processes that enrich rocks in oxides, such as ilmenite, are controversial. Current models include magmatic accumulation, crystallisation of veins from immiscible liquids and syntectonic differentiation. In this contribution, we investigate examples of oxide enrichment in both the oceanic and continental crust. The oceanic samples are of oxide gabbros (with up to 45 vol% oxides) from the Atlantis Bank oceanic core complex, Southwest Indian Ridge. The continental sample is from the Cattle Water Pass shear zone (with up to 20 vol% oxides) associated with the intracontinental Alice Springs Orogeny, central Australia. We argue for the occurrence of an open chemical system, with melt rock reactions as a key process involved in oxide enrichment in melt-fluxed shear zones. Our detailed microsUuctural characterisation reveals that oxides replace silicates and form interstitial grains, grain boundary films and low dihedral angles between silicates often making up an interconnected skeletal texture. Quantitative orientation data reveals that the oxides: 1) have limited internal deformation, 2) form clusters of grains that are connected in 3D, 3) have crystal faces matching die orientadon of the grain boundary of nearby newly crystallised diopside (oceanic sample) and 4) form part of the foliation defining assemblage with biotite (continental sample). This evidence suggests the oxides crystallised in the presence of melt and formed during melt-rock interaction. Syntectonic melt migration is known to result in low strain mi-crostructures in shear zones, as the strain is accommodated by the melt that existed in the deforming rock. This produces a high strain rock with silicate and oxide minerals that show limited internal deformation. Micro-chemical data shows major element variability in silicates and ilmenite at the thin section scale, supporting an open chemical system with local variability in both oceanic and continental settings. It further argues that syntectonic melt migration is important in oxide enrichment. Mineral chemistry data implies that the oceanic tectonic setting involved melt-rock interaction with fractionated gabbroic melt while the continental setting involved peraluminous granite melt driving mineral replacement and enrichment of oxides. We propose that deformation assisted reactive porous flow of near liquidus melt dirough rocks in any tectonic setting may result in melt-rock interaction induced crystallisation of oxides in preference to silicates and that with high time-integrated melt flux, the accumulation of oxides can be significant.
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    Petrogenesis and tectonic setting of A-type granites in the Babouri-Figuil Magmatic Complex (North Cameroon): Constraints from whole rock geochemistry, zircon U—Pb geochronology and Sr-Nd-Hf isotopes

    Afanga Archelaus Emmanuel BasuaChangqian MaKanouo Sylvestre Nguo
    30页
    查看更多>>摘要：The tectonics of the Central Africa Orogenic Belt (CAOB, in Cameroon, Chad, and the Central Africa Republic) is poorly constrained. For this reason, a study was carried out at the Cameroon-Chad border from biotite granite and hornblende-biotite massifs that intruded a gneiss-amphibolites basement of the NW Cameroon domain. Together, we report data on the whole-rock major and trace element compositions, mineral chemistry, and zircon U/Pb ages, supported by Nd-Sr-Hf isotopes. Both rocks are calc-alkaline, except for one sample with a Rittmann Serial Index (o) >3.5 (alkaline), high-K, and marginally peraluminous. The biotite granite is alkalic-calcic, whereas hornblende-biotite granite is alkalic. These rocks are characterized by high contents in SiC>2 (>69.85 wt%), Fe-number [FeOt/(FeOt+MgO)] (ferroan), TiO2/MgO, and low CaO, MgO, and P205 contents. Their penological and geochemical features, such as early formed sodic and iron-rich amphibole, late crystallized biotite, and high Ga/Al ratios, are consistent with A-type granite formed from F and CI enriched melts. Biotites compositions are close to those of annite, and amphiboles to ferro-amphibole, ferro-edenite, and ferro-gedrite types. The correlation for some major elements against Si02 is consistent with fractionation of K-feldspar, plagioclase, apatite, amphibole, biotite, Fe—Ti oxides, and MnO-rich phases under 882 to 967 °C, < 5 kb, and low .fC>2 (AQFM =-0.55) at shallow crustal levels between <1 and 7 Km. Zircon U/Pb geochronology indicates two magmatic episodes for the biotite granite (RBO-27 and SMO-8) 602 ±1.6 Ma, 602 ± 1.8 Ma, and (DBE-9) 707 ± 2.5, whereas hornblende-biotite granite (HRI-23) yielded 604 ±1.0 Ma, respectively. The younger group yielded moderately to notably depleted initial Hf and Nd values [eHf(t) =-3.4 to ±1.8, av. =-1.80; eNd(t) =-2.46 to-0.95, at 1.34 to 1.51 Ga], and low to moderate calculated initial Sr values (87Sr/86Sr(i) = 0.66781-0.70287). The older A-type granite yielded highly depleted initial Hf values (eHf(t) = ± 7.74 to ±9.30). The strong positive Pb anomaly for these granites indicates involvement of either crustal component or melt from the subducted slab. Given that the obtained initial Hf and Nd values are enriched relative to the depleted mantle (eHf(t) —hl4; eNd(t) —h6.5), it therefore, suggest that these granites could be derived from low-degree partial melting of a juvenile crust extracted from the mantle.
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    Continental crust recycling in ancient oceanic subduction zone: Geochemical insights from arc basaltic to andesitic rocks and paleo-trench sediments in southern Tibet

    Long ChenYong-Fei ZhengZi-Fu Zhao
    18页
    查看更多>>摘要：The recycling of continental crust via trench sediment subduction and subduction erosion is well established in modern oceanic subduction zones, and tiiis is fundamental to subduction zone magmatism and mantle heterogeneity. Although a similar process likely took place in ancient oceanic subduction zones, its evaluation is difficult because most geological records were erased by subsequent continental collision and post-collisional reworking processes. To address this issue, a comprehensive geochemical study was conducted for Mesozoic basaltic to andesitic plutonic rocks and surviving contemporaneous paleo-trench sediments from the Himalayan orogen in southern Tibet. These plutonic rocks show variably enriched Sr-Nd-Pb-Hf isotope compositions with t'Nd(t) values varying from-5.6 to +4.7, which are neither positively nor negatively correlated with Mg# values. This indicates that they were not affected by crustal contamination but rather inherited from heterogeneous magma sources. There are consistently continuous variations in the Sr-Nd-Pb-Hf isotope space among the paleo-trench sediments, the arc plutonic rocks, and basaltic rocks from the Yarlung Tsangpo ophiolite suites, indicating that the binary mixing between the paleo-trench sediment-derived melts and the ambient mantle dictated the radiogenic isotope compositions of the arc igneous rocks. These qualitative interpretations are further verified by quantitative geochemical examination of the compositional links in Nd and Hf isotopes between the paleo-trench sediments and arc rocks. As a consequence, the present study has for the first time identified and systematically demonstrated the key role of paleo-trench sediments in constraining the continental crust recycling as well as the origin and compositional heterogeneity of arc igneous rocks above ancient oceanic subduction zones.
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    Crystallisation and fast cooling of the (meta)gabbro from the Chenaillet ophiolite (Western Alps): In-situ U—Pb dating of zircon, titanite, monazite and xenotime in textural context

    Emilie BruandChristian NicolletJean-Louis Paquette
    16页
    查看更多>>摘要：The Chenaillet ophiolite, South of Montgenevre (France), represents a preserved portion of the alpine ocean. The gabbros form lenticular bodies 50 to 200 m thick and a few hundreds of meters wide. They are intrusive in the serpentinites and often overlain by pillow-lavas. Plagiogranite/albitite veins are rare and volumetrically negligible. The petrology of the gabbro s.s. show great details of the progressive cooling of the massif, clinopyroxene are often rimmed by brown to green amphibole +/-ilmenite/titanite. Thermometry on these amphibole assemblages indicates a retrograde temperature evolution from late magmatic to subsolidus, between 950 and 900 ° C, 800-750 ° C, 600-500 ° C. In a shear zone, a string of titanite associated with monazite and xenotime +/-ilmenite is located in a millimetric greenschist layer whose temperature is estimated at 600-550 CC. Accessories chemistry in mis sample allowed in situ dating by LA-ICPMS on thin section in textural context. In die same sample, we performed U—Pb zircon dating, constraining the magmatic age, as well as U—Pb monazite, xenotime and titanite dating for the metamorphic evolution. In a Tera-Wasserburg diagram, monazite, xenotime and titanite yield 161.3 +/-4.0 Ma, 161.5 +/-2.4 Ma and 158.4 +/-2.3 Ma lower intercept ages, respectively, while the zircon from the same sample indicates an age of 161.0 +/-0.8 Ma. Finally, zircon dated in an albitite vein also yields a 161.8 ± 1.7 Ma age. These similar ages within the analytical uncertainties show that mag-matism and metamorphism were concomitant. Thermal modelling predicts that a 150 m thick gabbroic massif crystallizes in a few hundred years and cools down in several thousand years. This is consistent with the ages reported in our study showing that the retrograde metamorphism in this massif records fast cooling. The development of this retrograde metamorphism required multiple fluid infiltrations during cooling. In the studied samples, 4 types of titanites could be distinguished based on their chemistries. 3 types variously depleted in REE are interpreted as related to water-driven fluids while another type has up to 10% REE, Y and Nb and suggest a possible felsic source. The two potential sources of water promoting this fast cooling are sea water-derived fluids and magmatic fluids exsolved from felsic veins.
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    Metal mobilization and precipitation in a Sn-W skarn system, Gejiu Sn district, China

    Rolf L. RomerJohannes GlodnyRong Xu
    20页
    查看更多>>摘要：In the Gejiu Sn-Cu polymetallic district, skarns with or without mineralization developed both at the contact zones between a granitic intrusion and carbonate-rich wall rocks and within carbonate-rich wall rocks. We analyzed the chemical and Sr, Nd, Pb, Li, and B isotopic compositions of mineralogically different zones in two hand specimens, i.e., a banded carbonate and a distal skarn with distinct reaction zones, to interpret chemical and isotopic variation in terms of (i) temporal compositional changes of skam-forming fluids and (ii) reaction-induced exchange between fluids and carbonate wall rocks. The banded carbonates have seen little fluid-mediated exchange and therefore the compositions of the various bands reflect a two-component system of carbonates and detrital silicates. The compositional range of the banded carbonates represents the compositional heterogeneity of the protoliths of the distal skarn that experienced a pronounced impact by metasomatic fluids. The studied distal skarn is characterized by the vein-like aquifer (Zone I), reaction zones (Zone II and Zone III), and the carbonate aquitard (Zone IV). As evident from the distal skarn, the chemical and isotopic compositions of skarn-forming fluids change over time due to (i) processes in the fluid sources, (ii) variable contributions of different fluid sources, and (iii) reaction-induced exchange with the rocks bordering the aquifer, leading to selective addition or removal of elements from the fluid. During early interaction between aquifer fluids and its wall-rocks, the aquitard is partially consumed and the chemical and isotopic signature of the newly-formed minerals in the reaction zone represent a mixture of material derived from either fluids or wall rock. Once formed, the reaction zones largely protected the aquitard from further interaction with fluids. The Sr-budget of zones I to IV is dominated by Sr derived from the carbonate aquitard and redistributed during the consumption of older reaction zones. The Pb, Li, and B budgets of zones I to III are dominated by materials derived from the fluids. In contrast, the Nd isotopic compositions in zones II to III fall between the values of the carbonate aquitard and the aquifer. The ore elements occur in different zones, i.e., (i) scheelite, sphalerite, pyrite, and chalcopyrite precipitate in the aquifer and (ii) Sn substitutes in calc-silicates at the reaction front between zones II and III and is present in late fractures in Zone III. Thus, ore elements may have been transported by different skarn-forming fluids. The chemical and isotopic signatures of reaction zones do neither strictly reflect the nature of the reactive fluid nor reflect simple bulk mixing between fluid and wall rock, as (i) reaction-induced volume decrease may allow later fluids to infiltrate into earlier reaction zones, (ii) the fluid may be depleted in elements that have been scavenged from the fluid before, (iii) some elements having low compatibility in minerals of the reaction zones may have strongly partitioned into the fluid and are lost from the system. Although reaction zoning provides information about the nature of the reactive fluids, the mineralogically-controlled selective behavior of trace elements, i.e., most ore elements, does not allow to use the ore element content in skarn as a vector to mineralization.
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    Pre-Late Eocene position of the Luchun-Jinping microblock in western Yangtze Craton: Constraints from Eocene-Oligocene lamprophyres in southeastern Tibet

    Yuan-Chuan ZhengZeng-Qian HouYang Shen
    18页
    查看更多>>摘要：The tectono-magmatic history of the Luchun-Jinping microblock and its possible affinity with the Yangtze Craton are important elements for the reconstruction of Cenozoic plate tectonics in southeastern Tibet. In order to constrain the affinity and decipher the pre-Cenozoic paleopositon of the Luchun-Jinping microblock, we focused on the petrogenesis of Eocene-Oligocene lamprophyres in the Luchun-Jinping microblock. The lamprophyres yield zircon U—Pb ages of 34.7-33.3 Ma and exhibit potassic-ultrapotassic features with elevated K2O/Na2O (1.4-4.0) ratios. They are characterized by high concentrations of compatible elements (e.g., Cr = 187-692 ppm, Ni = 31-218 ppm), large-ion-lithophile elements and light rare-earth elements enrichment, high-field-strength elements depletion, and high radiogenic isotopic values, i.e. (87Sr/86Sr)/ = 0.7063-0.7078 and eNd(t) =-3.9 to-2.4. Combined with the low Nb/U ratios, these features suggest that the lithospheric mantle source was metasomatized by subduction-related fluids beneath the Luchun-Jinping microblock. The relatively high Rb/Sr ratios and high heavy rare-earth element contents indicate that these lamprophyres were derived from partial melting of a phlogopite-bearing lherzolite within the spinel stability field. The parental magmas have experienced fractional crystallization of olivine and clinopyroxene during emplacement. Comprehensive comparisons between the lamprophyres from the Luchun-Jinping microblock and the potassic-ultrapotassic mafic rocks from the western Yangtze Craton indicate that the Luchun-Jinping microblock can be regarded as a dismembered part of the western Yangtze Craton due to continental extrusion and Cenozoic sinistral displacement. The compositional trends of the potassic-ultrapotassic mafic rocks suggest that the palaeogeographic position of the Liichun-Jinping microblock was near the Dali area (west of the Binchuan) and close to the Jinshajiang suture zone before the Cenozoic.
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    Xigaze ophiolite (South Tibet) records complex melt-fluid-peridotite interaction in the crust-mantle transition zone beneath oceanic slow-ultraslow spreading centers

    Lian-Rong TianJian-Ping ZhengQing Xiong
    15页
    查看更多>>摘要：The formation and evolution of the crust-mantle transition zone (CMTZ) under oceanic slow-ultraslow spreading centers, compared with the well-documented examples that developed under fast spreading centers, remain largely unknown due to the lack of suitable targets. In this study, we systematically examine the CMTZ of the Xigaze ophiolite in die Yarlung Zangbo suture zone (south Tibet). This ophiolite represents a rare lithospheric fragment produced in oceanic slow-ultraslow spreading settings. We conduct detailed field mapping and petrological as well as geochemical studies on the CMTZ in the Dazhuka massif, the easternmost segment of die Xigaze ophiolite. Our aim is to characterize the lithological architecture and associated melt-fluid-peridotite interaction history of the CMTZ. The CMTZ (-1800 m thick) in the Dazhuka ophiolite consists of 4 subzones with complex lithological associations. They range successively from clinopyroxene-rich harzburgite with weak metasomatism and without amphibole in Zone 1 (the bottom) to clinopyroxene-poor harzburgite with strong melt impregnation and more amphibole in Zone 3 (die upper part, with Zone 2 transitional harzburgite in between), all cut by a wealth of dyke rocks (gabbro, dolerite and dunite). The AI2O3 contents decrease consistently in whole-rock (2.97-0.57 wt%), orthopyroxene (4.99-0.85 wt%) and clinopyroxene (5.25-1.22 wt%) for the harzburgites from Zones 1-3, contrary to the trends for the spinel Cr# (0.17-0.65) and clinopyroxene Li/Y, and bulk rare earth element (REE), Pb and Sr contents. The plagioclase-bearing peridotite (commonly enclosed in layered gabbro) in Zone 4 (the top) has remarkably elevated TiO2 contents (0.12-0.34 wt%) and Cr# values (0.45-0.53) in spinel, suggesting equilibration and strongest interaction with MORB-like melts among die CMTZ. The troctolite in Zone 4 has olivine Mg# (80.6-84.2) and NiO (0.37-0.45 wt%) and spinel Cr# (0.68-0.73). The fluid-mobile elements (U, Pb, Sr and Li) in both whole rock and clinopyroxene as well as amphibole abundances increase consistently from Zones 1-4, implying that die interaction of the mantle rocks with hydrodiermal fluids became increasingly intensive from bottom to top of the CMTZ. These vertical variations collectively suggest die complex melt-fluid-peridotite interaction during die upward movement of deeply sourced melts and the downward penetration of seawater. We propose that such interactions are ultimately controlled by the relative slow spreading rates, where the less melt supply and very thin or even missing oceanic crust facilitate the downward seawater injection and enhance pervasive fluid metasomatism. This may represent a suitable explanation for the CMTZ with intriguing lithological and chemical heterogeneity under many other active or fossil oceanic slow-ultraslow spreading environments.
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    Casting a vote for shifting the Statherian: Petrogenesis of 1.70 and 1.62 Ga mafic dykes in the North China Craton

    Chong WangPeng PengRoss N. Mitchell
    17页
    查看更多>>摘要：The Statherian Period (1.8-1.6 Ga), currently the final period of the Paleoproterozoic Era, is named "stable, firm" for the interval following orogenesis (the Orosirian Period) that culminated in the assembly of likely Earth's first supercontinent, Columbia. Thus, the Paleoproterozoic-Mesoproterozoic boundary is supposed to demarcate increased continental "stability", defined as a transition from orogenesis and arc magmatism to unmetamor-phosed shallow-water cover sequences and intraplate magmatism. It has become apparent, however, that (i) the assembly of Columbia appears to be highly diachronous and (ii) many stable cover sequences once mapped as Mesoproterozoic appear to pre-date 1.6 Ga quite significantly. These discrepancies have led to die recent suggestion by a large group of Precambrian geologists that the Statherian Period belongs more to the Mesoproterozoic Era and thus that the end of the Paleoproterozoic Era should be redefined at ca. 1.8 Ga. North China Craton is one such craton that has pre-1.6 Ga stable sedimentary covers, but dating the base of the basins have proven difficult and thus other indicators of continental stability must be sought. Here we present new geochronological and geochemical data from two dyke swarms at 1.70 and 1.62 Ga from western Shandong Province (Luxi area) of the North China Craton that can be used to constrain the tectonic setting at this critical interval in question. Precise U-Pb SIMS analysis yields baddeleyite 207Pb/206Pb ages of 1700 ± 5 Ma (n = 9, MSWD = 0.14) and 1620 ± 4 Ma (n = 14, MSWD = 0.56). The new dated ca. 1.62 Ga Xiaobeizhuang dyke (330° trending, >15 m wide) belongs to the coeval Taishan dyke swarm. The ca. 1.70 Ga swarm, newly discovered in the North China Craton, is referred to as Ximaiyao dyke swarm (340-345° trending, 0.4-13 m wide). Paleogeographic affinities of the North China Craton with the Sao Francisco Craton in super-continent Columbia are considered in light of this newly identified swarm. Given that both dyke swarms pre-date 1.6 Ga and that their geochemistry implies intraplate magmatism, our new data support the suggestion that this time is more Mesoproterozoic than Paleoproterozoic in character. These pre-1.6 Ga intraplate dyke swarms, including the 1.78 Ga, the 1.73 Ga, and later 1.70 Ga and 1.68-1.62 Ga swarms, thus provide local support from North China Craton for redefining the base of the Mesoproterozoic Era as ca. 1.8 Ga and including the Statherian Period, consistent witii the ages of platform cover sequences.
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    Sanukitoid crystallization relations at 1.0 and 0.3 GPa

    Daniel Gomez-FrutosAntonio Castro
    13页
    查看更多>>摘要：Sanukitoid (vaugnerites, appinites, durbachites) intrusions have been described in multiple locations around the world. Initially thought as an only basic-intermediate series, increasing interest in these rocks have pointed to the conclusion that they represent a whole differentiation series. A distinct geochemistry with high MgO, LILE and LREE (specially K, Ba and Sr) and depletion in CaO differentiates them from the standard calc-alkaline batholiths they appear associated with. While some of such features may be inherited from the metasomatized mantle source, no consistent explanation has been given for the singular geochemistry of the series. Accordingly, we propose two initial hypotheses: (1) the sanukitoid array is due to the existence of a cotectic line that determines the behaviour of die system; (2) the sanukitoid array does not represent a cotectic series, but the result of a ubiquitous process that deviates the rocks from their ideal differentiation trend. To test the thermodynamic behaviour of the system piston cylinder experiments were conducted at different temperatures in two pressure series appropriate for lower and upper crust differentiation (1.0,0.3 GPa and 1100 °C, 1050 °C, 1000 °C, 900 °C). Two starting compositions are used: a sanukitoid enclave from Los Pedroches badiolith (SW Spain) and a hydrous synthetic glass. Experimental results show that, while 1.0 GPa runs follow the calc-alkaline cotectic, 0.3 GPa runs follow a line that coincides with the sanukitoid series, proving die existence of a low pressure cotectic line. However, lack of cumulates at shallow depths and low water contents in die fractionated magmas (the granites) suggest that a complementary mechanism must be involved. The observed geochemical features may also be accounted for by contamination by entrainment of orthopyroxene from the source or early magmatic cumulates. This is supported by the showcased crystallization pattern, with orthopyroxene representing the liquidus phase, die presence of amphibole clots in tiiese rocks, and the presence of autoliths with abundant orthopyroxene in sanukitoid intrusions. Additionally, the existence of a low pressure cotectic system induces the re-equilibration and digestion of such entrained ordiopyroxene in die liquid when reaching shallow deptiis, leaving a seemingly homogeneous liquid. The two discussed mechanisms are valid, compatible, complementary and account for die geochemical trend shown by die series.
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