235. CHAPTER 8 Sut·111ARY Detailed petrographic, mineralogical, chemical and isotopic studies of the previously poorly documented southern and southeastern portionsof the composi tc Permo-Carboniferous :~ew England Batholi th have delineated a number of distinct igneous suites. The Hillgrove Suite, from which this study arose, has been restricted from its original definition, which included both mafic and felsic rocks, to those plutons which constitute predominantly the south­ eastern flank of the batholitic "core" (Fig. 2.1) and which display features characteristic of S-type granitoids (Chapter 3). This suite is intruded into thick sequences of radiometrically dated predominantly Devonian and Carboniferous volcanogenic greywackes, siltstones and shales. Relatively thin veneers of locally fossiliferous Early Permian sediments unconformably overlie these older sediments. Recognition of these unconformities has been exceedingly difficult because bedding and exposure are generally poor, lithologies are very similar, but more importantly, because the entire sequence, including the younger beds, was strongly deformed during the early­ and mid-Permian. A sequence of major tectonic events commenced in New England during the early- to mid-Carboniferous which resulted in the generation of voluminous quantities of granitoids and related rocks. The first of these events is considered to have been a downbuckling of the moderately thick sedimentary pile into a PIT environment in which partial melting occurred (- 680- 700°C). The composition of the intermediate to felsic volcanogenic detritus was such that substantial quantities of minimum melt were produced. A temporary cessation of compressive forces allowed the melts to coalesce, then move upwards despite only small density contrasts between the magma and the relatively plastic metasediments. The extent to which these melts moved away from their sites of generation appears to have been related to their degree of water-saturation, the composition of the enclosing metasediments and the distribution of compressive forces. Thus, granitoids of the Hillgrove Suite which display minimal contact metamorphic effects and are associated with major faults, e.g. the Hillgrove, Enmore and Kilburnie Adamellites, rose higher in the crust than those which are surrounded by a mantle of high-grade metamorphics, e.g. the Argyll, Tia and Abroi Granodiorites. 236. A derivation of the Carboniferous Hillgrove Suite granitoids from the exposed Devonian greywackes and intercalated pelitic rocks is consistent with mineralogical and chemical data, with the time gap (60- 100 m.y.) between sedimentation and granitoid formation noted elsewhere, and with the fact that these possible source rocks had essentially the same 87S r J86Sr ratios at - 320 m.y. as the initial 87Sr /86Sr ratio of the granitoids. However, RE and other selected trace element data preclude a derivation for the Hillgrove Suite by partial melting of the sediments which surround the granitoids at the present erosion level but suggest that similar, though less REE-enriched sediments such as those of the Tamworth Trough, may be an essential component of the source. Comparison of the granitoid and mineral compositions with experimental phase equilibrium studies suggest that the majority of plutons were generated and crystallized between 4 and 7 kb F1120. However, final emplacement (or uplift) may not have occurred for some time, as shown by the consistently younger biotite ages compared with total-rock ages. Variations in some of the biotite ages are concluded to reflect differences in the amount of vertical movement of the granitoids from the zone of magma generation. The association of the calc-alkaline mafic intrusive complexes with the Hillgrove Suite granitoids appears to be related to the compressive phase which accompanied emplacement of the latter. Sr isotopic ages and field relations suggest that the time period between the emplacement of the granitoids and of the calc-alkaline complexes may have been small (- 10-20 m.y.). Petrographic, mineralogical, chemical and Sr isotopic data suggest that these complexes are products of contaminated tholeiitic basaltic magma. During the early Permian a second and significantly more-extensive downbuckling of the somewhat thicker New England sedimentary pile occurred, bringing it into close proximity with the upper mantle. The higher temperatures triggered a complex sequence of progressive fractional melting events which yielded felsic granitoids, adamellites, granodiorites, diorites, microtonalites and monzonites. These comprise the New England Suite. The sequence of fractional melting, accumulation of residual ferromagnesian phases, and remelting of the latter, produced an extreme diversity of textural, mineralogical and chemical features, previously interpreted as resulting from partial melting of igneous source rocks or by fractional crystallization of enriched upper mantle material. Sr isotopic data provide convincing evidence to support my 237. model that the plutons of this Suite are consangineous,having been derived from essentially the same type of sedimentary source material which yielded the plutons of the Hillgrove Suite, although specific areas within this source material may have differed slightly in chemical composition. The Sr isotopic data furthermore provide sound evidence that it is highly unlikely for the source material of either the J:\eH England or Hillgrove Suites to have contained a Precambrian component or that Precambrian continental crust underlies the Ne\"r England Fold Belt. The downbuckling of the crust during the early Permian to produce the New England Suite, and its subsequent "rebounding", is coincident with major diastrophism over the entire New England Fold Belt. Severe compression along the locus of the Hillgrove Suite especially caused extensive folding and faulting, superposing a well-developed, sometimes bi-directional, tectonite fabric onto an earlier fabric in these granitoids. In addition, the di~trophism during this period may possibly have caused a further slight uprise of some Hillgrove Suite plutons, so that they became even more effectively isolated from any subsequent periods of lower crustal magma generation and intrusion. This is suggested by the older biotite ages of certain "high-level" plutons compared with those plutons which remained close to their site of generation. The major crustal fracturing in the area of the Hillgrove Suite, during the period accompanying "rebound" and the emplacement of the relatively deeply generated New England Suite, permitted intrusion of some of the upper mantle material which provided the heat source for the Permian melting events. This upper mantle material, represented by the tholeiitic intrusive complexes associated with the Hillgrove Suite, is manifest at the present level of exposure, as comparatively small igneous complexes. The primitive chemical and isotopic characteristics of the olivine gabbros in these complexes has generally been preserved despite their emplacement through rather thick crust. However, minor contamination of the gabbroic magma by crustal material during the early stages of its ascent, followed by crystal accumulation and intense fractional crystallization at various conditions of f02 and aH20, produced a wide range of derivatives. The early and middle stages of fractionation are characterized by absolute iron-enrichment; this is expressed mineralogically by the crystallization of ilmenite in the dioritic rocks and by very iron-rich amphiboles and biotites in the granophyres. Although the adamellites and more-felsic rocks follow a trend which is dominated by alkali enrichment, 238. their chemistry and mineralogy reflect a history of fractional crystallization rather than one of partial melting of country rocks. A group of isolated (7) late Carboniferous granitoids associated with the Peel Fault display features which distinguish them from other granitoids of the New England Batholith. They are characterized by low contents of K20 and related incompatible elements, very low initial 87Sr /86Sr ratios and they have textures which indicate rapid crystallization from a total melt. These criteria are consistent with a model in which chemically and isotopically primitive but amphibolitized ocean-floor basalts, believed to have formed in a marginal sea, are partially melted as the marginal sea floor was being subducted, presumably at the arc-trench gap. This model is further consistent with the occurrence of high-pressure low-temperature metamorphic rocks along the Peel Fault. 239. REFERENCES Albuquerque, C.A.R.de, 1973. Geochemistry of biotites from granitic rocks, northern Portugal. Geochim. Cosmochim. Acta, }l, 1779-1802. Albuquerque, C.A.R.de, 1974. Geochemistry of actinolitic hornblendes from tonalitic rocks, northern Portugal. Geochim. Cosmochim. Acta, ~, 789-803. Albuquerque, C.A.R.de, 1977. Geochemistry of the tonalitic and granitic rocks of the Nova Scotia southern plutons. Geochim. Cosmochim. Acta, 41, 1-13. Albuquerque, C.A.R.de, 1978. Rare earth elements in 'younger' granites, northern Portugal. Lithos, 11, 219-229. Andrews, E.C., 1900. Report on the Hillgrove Goldfield. Mineral. Resour. N.S.W., ~. Andrews, E.C., 1904. The geology of the New England Plateau. Part I - Physiography. Rec. Geol. Surv. N.S.W., I, 281-300. Andrews, E.C., 1905a. The geology of the New England Plateau, with special reference to the granites of northern New England. Part I - Physiography. Rec. Geol. Surv. N.S.W.,