Barberton Granitoid-greenstone Terrain Research Articles

Thermal evolution of the Stolzburg Block, Barberton granitoid-greenstone terrain, South Africa: Implications for Paleoarchean tectonic processes

The thermal state of the crust controls its rheology and is therefore an important variable in understanding Archean tectonic processes. Thermochronology is a useful tool to track the long-term thermal evolution of the crust, and U-Pb in apatite, with an effective closing temperature for volume diffusion of Pb of ca. 400–550 °C, is useful in constraining the temperature-time history of igneous and metamorphic rocks, when apatite contains sufficient U. The Paleo- and Mesoarchean represent time periods in Earth’s history where there is little certainty about the geodynamic processes that controlled the movement of crustal blocks. In this study on the Paleoarchean Stolzburg Block of the Barberton Granitoid Greenstone Terrain (BGGT), South Africa, we report new U-Pb apatite and zircon ages, as well as Lu-Hf zircon data for several TTG plutons that are considered to represent the type of felsic crust that formed the nucleus of the proto-Kaapvaal craton. Zircons from the Stolzburg, Theespruit and Weergevonden plutons provide apparent crystallization ages of ca. 3451 Ma in agreement with earlier studies. Felsic dykes concentrated in the deformed margins of the Stolzburg and Theespruit plutons yielded zircon crystallization ages of ca. 3215 Ma, postdating the main 3230–3225 Ma accretionary event in the belt. Zircon εHft values were determined to be +1.2 to −0.8. in the 3450 Ma plutons and +0.4 to −2.0 in the 3215 Ma dykes. U-Pb apatite thermochronology for the plutons shows igneous ages of ca. 3445 Ma, but also and multiple subsequent events of apatite growth or recrystallization at ca. 3210, 3105 and 2820 Ma. The preservation of this range of apatite dates indicates that no post-emplacement heating of the granitoids above ca. 600 °C has occurred. The Hf isotopic data show that the ca. 3.2 Ga dykes are less evolved than their host TTG plutons and are not produced by melt extraction following anatexis of the latter or their equivalents. Collectively, this evidence demonstrates that the Stolzburg, Theespruit and Weergevonden plutons have not undergone anatexis and have not been mobilized as anatectic domes at ca. 3.2 Ga, as proposed by some tectonic models

CHERTS OF THE BARBERTON GREENSTONE BELT, SOUTH AFRICA: PETROLOGY AND TRACE-ELEMENT GEOCHEMISTRY OF 3.5 TO 3.3 GA OLD SILICIFIED VOLCANICLASTIC SEDIMENTS

Research Article| December 01, 2013 CHERTS OF THE BARBERTON GREENSTONE BELT, SOUTH AFRICA: PETROLOGY AND TRACE-ELEMENT GEOCHEMISTRY OF 3.5 TO 3.3 GA OLD SILICIFIED VOLCANICLASTIC SEDIMENTS A. HOFMANN; A. HOFMANN Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: ahofmann@uj.ac.za Search for other works by this author on: GSW Google Scholar R. BOLHAR; R. BOLHAR Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa School of Earth Sciences, University of Queensland, Brisbane, Qld 4072, Australia, e-mail: rbolhar71@gmail.com Search for other works by this author on: GSW Google Scholar B. ORBERGER; B. ORBERGER ERAMET, 1 Avenue Albert Einstein, 78190 Trappes, France, Université Paris Sud XI, UMR IDES 8148, Département des Sciences de la Terre, Bât 504-509, 91405 Orsay Cedex, France, e-mail: beate.orberger@erametgroup.com Search for other works by this author on: GSW Google Scholar F. FOUCHER F. FOUCHER Centre de Biophysique Moléculaire, CNRS, Rue Charles Sadron, 45071 Orléans, France, e-mail: frederic.foucher@cnrs-orleans.fr Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2013) 116 (2): 297–322. https://doi.org/10.2113/gssajg.116.2.297 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. HOFMANN, R. BOLHAR, B. ORBERGER, F. FOUCHER; CHERTS OF THE BARBERTON GREENSTONE BELT, SOUTH AFRICA: PETROLOGY AND TRACE-ELEMENT GEOCHEMISTRY OF 3.5 TO 3.3 GA OLD SILICIFIED VOLCANICLASTIC SEDIMENTS. South African Journal of Geology 2013;; 116 (2): 297–322. doi: https://doi.org/10.2113/gssajg.116.2.297 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract A suite of green and carbonaceous black chert interbedded with submarine volcanic rocks from several stratigraphic levels of the Onverwacht Group of the Barberton greenstone belt, from the top of the ~3.48 Ga Komati Formation to the base of the ~3.26 Ga Fig Tree Group have been investigated petrologically and geochemically. Provenance analysis was undertaken using immobile trace element ratios in comparison with potential source rocks from the Barberton granitoid-greenstone terrain. Raman spectroscopy was used to better characterise carbonaceous matter in different chert varieties. Green cherts consist predominantly of silicified mafic to ultramafic volcaniclastic material. Black cherts contain volcaniclastic and/or epiclastic material of ultramafic to felsic composition, admixed with carbonaceous matter. In several cases, the clastic sediment is compositionally distinct to the volcanic substrate, suggesting that it represents volcanic material of relatively distal sources. Soon after settling out of the water column, lithification due to silicification took place. It gave rise to excellent textural preservation, but strongly modified the mineralogical and chemical composition of the seafloor sediments. Zircon, Cr-spinel and rare phosphate minerals are the only primary minerals remaining, while clastic grains have been largely replaced by a fine intergrowth of microquartz, Ti-oxide and K-bearing mica. Chemical changes are similar to those reported previously for silicification of volcanic rocks from the Barberton belt and are consistent with low-temperature hydrothermal processes common to the Palaeoarchaean seafloor environment. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Water-present eclogite melting to produce Earth's early felsic crust

The geochemistry of well preserved Archaean Tonalite–Trondhjemite–Granodiorite (TTG) rocks, such as the trondhjemites of the Meso-Archaean Barberton granitoid–greenstone terrain in South Africa, provides insight into the origins of Earth's early felsic continental crust. This is particularly well demonstrated by the high pressure (HP)-type of Archaean TTG magmas, where the geochemistry requires that they are formed by high-pressure melting of a garnet-rich eclogitic source. This has consequently been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. Most of the experimental data relevant to TTG genesis has been generated by studies of fluid-absent melting of metabasaltic sources. However, water drives arc magmatism within Phanerozoic subduction zones and thus, understanding the behaviour of water in Archaean subduction zones, may have considerable value for understanding the genesis of HP-type Paleo- to Meso-Archaean trondhjemites. Consequently, this study investigates the role of high-pressure water-present melting of an eclogite-facies starting material, in the production of Paleo- to Meso-Archaean HP-type TTG melts. Water-saturated partial melting experiments were conducted between 1.9 and 3.0GPa; and, 870°C and 900°C. The melting reaction is characterised by the breakdown of the jadeite molecule in the clinopyroxene, together with quartz and water, to form melt in conjunction with a less sodic clinopyroxene: Qtz+Cpx1+Grt1+H2O=Melt+Cpx2+Grt2 and produced melt compositions that, if allowance is made for the low Mg# and almost K2O-free character of the starting material, are an excellent major element match with HP-type TTG compositions. In two of the experimental run products, melt segregated efficiently from residual crystals, allowing for the measurement of a full range of trace elements, via Laser Ablation Inductively Coupled Plasma Mass Spectroscopy (LA-ICP-MS). The experimental glasses produced are Light Rare Earth Element (LREE) (24–44ppm La; 50–86ppm Ce) and Zr (101–228ppm) enriched; and Heavy Rare Earth Element (HREE) (0.4ppm Yb; 50.9 La/Ybn), Y (2.2–3.2ppm; 82–142 Sr/Y), Sm (2.1–4.3ppm; 40.9–50.8 Zr/Sm) and Nb (1.7–2.8ppm) depleted compared to the DSE4 starting material. Thus, the REE element compositions of the experimental glasses are also similar to HP-type TTG compositions, and in more particular they are strikingly similar to Meso-Archaean Barberton trondhjemites. Additionally, we propose that due to Cpx being a major reactant, Ni and Cr contents (4–29ppm Ni and 43ppm Cr) of the glasses which where analysed for these elements, are within the compositional range displayed by HP-type Archaean TTG. This study suggests that the geochemistry of HP-type Archaean trondhjemites may reflect high pressure water-present partial melting of an eclogite facies metabasaltic source. Consequently, we propose that water-present melting of an eclogitic source is a viable mechanism for the genesis of this component of Paleo- to Meso-Archaean TTG crust.

3230–3200 Ma post-orogenic extension and mid-crustal magmatism along the southeastern margin of the Barberton Greenstone Belt, South Africa

The Barberton Granitoid-Greenstone Terrain (South Africa) preserves a complex and protracted evolution involving several events of magmatism and terrain accretion along convergent tectonic boundaries. Recent studies propose that the main period of tectonic accretion and arc-related magmatism is linked to a system of divergent subduction zones above which voluminous TTG magmas were emplaced between ca. 3236 and 3227 Ma. Our structural and LA-ICP-MS U–Pb geochronology study along the southeastern margin of the Barberton Greenstone Belt (BGB) ties the waning stages of this TTG magmatism to a short (ca. <30 Ma) period of mid-crustal extension, between 3228 and 3205 Ma. We document a major NE-trending detachment that juxtaposed upper sequences (Moodies Group clastic sediments) against mid-crustal 3418 ± 10 Ma amphibolite-facies rocks of the base of the greenstone belt (Onverwacht Group rocks). Several granodiorite bodies - intruded along this detachment - contain well-preserved (syn- and post-magmatic) fabrics that are demonstrably related to extensional shearing and exhumation. Field observations and U–Pb zircon data from these granitoids are consistent with the deformation taking place at 3228 ± 10 Ma – contemporaneous with the voluminous (3236–3227 Ma) TTG magmatism in the northwestern margin of the BGB. The timing of the granodiorite emplacement also constrains a minimum age for the deposition of the Moodies Group clastic sediments, which for much of the southern and southeastern parts of the BGB must have happened before ca. 3228 Ma. 3205 ± 9 Ma subvolcanic dykes intruded into the granodiorite complex indicate that the period of exhumation and cooling of the crystalline rocks along the extensional detachment was relatively short (<30 Ma), between 3228 and 3205 Ma. Our observations combined with previously published structural data from the northwestern and southern margin of the belt suggest that the main mechanism of large-scale infolding of the supracrustal strata was shortly followed by the extension-related magmatism and subsequent, solid-state diapiric movement of the arc-related plutons.

Continental growth and convergence-related arc plutonism in the Mesoarchaean: Evidence from the Barberton granitoid-greenstone terrain, South Africa

U–Pb zircon ages from trondhjemitic gneisses of the Badplaas domain, located in the southwestern parts of the Mesoarchaean Barberton granitoid-greenstone terrain of South Africa, document a hitherto largely unrecognized plutonic event. The geochronological results indicate the addition of juvenile, felsic crust over a period of 60 Myr between ca. 3290 and 3230 Ma and prior to the main collisional event in the granitoid-greenstone terrain at 3230 Ma. The timing and duration of plutonism, together with the structural and compositional heterogeneity of the Badplaas domain suggest that the Badplaas domain represents part of a convergence-related magmatic arc. On a regional scale, the spatial and temporal relationships between plutonism, metamorphism and deformation are interpreted to preserve an almost complete inventory of a Mesoarchean arc-trench system. This includes (1) the 3290–3230 Ma convergence-related magmatic arc, (2) a largely coeval deposition of the back-arc type volcano-sedimentary 3260–3225 Ma Fig Tree Group of the Barberton greenstone belt, (3) relics of the underplated crust preserved in high-P, low-T rocks to the immediate east of the magmatic arc, juxtaposed against (4) high-T medium-P rocks of the overlying plate along the major crustal structure of the Inyoni shear zone. These findings corroborate the notion that thermal and rheological conditions on the early Earth supported, at least locally, crustal growth and the extraction of buoyant felsic crust along convergence-related magmatic arcs.

Exhumation of Mesoarchean TTG gneisses from the middle crust: Insights from the Steynsdorp core complex, Barberton granitoid-greenstone terrain, South Africa

The processes leading to the assembly of supracrustal greenstone sequences and plutonic rocks of the trondhjemite-tonalite-granodiorite (TTG) suite, the two main building blocks of all Archean cratons, form a central aspect of our understanding of early continental growth. Key metamorphic and structural data are presented from granitoid-greenstone contacts in the Steynsdorp dome, the oldest and one of the best preserved parts of the Mesoarchean Barberton granitoid-greenstone in South Africa. Pressure and temperature estimates of P = 10–13 kbar and T = 640–660 °C from supracrustal rocks of the Steynsdorp dome indicate the burial of these rocks to depths >30–40 km during the main phase of collisional (D2) tectonics recorded in the terrane. Peak metamorphic assemblages define flattening-type fabrics, indicating crustal stacking of cool, rigid crust. The subsequent retrogression of rocks is associated with constrictional fabrics that are interpreted to have formed in response to the orogen-parallel extrusion and exhumation of rocks. The progressive retrogression of rocks and associated fabric development, unidirectional lineation and fold plunges, and consistent granitoid-up, greenstone-down kinematic indicators point to the exhumation of the TTG gneisses along an extensional detachment below the low-grade Barberton greenstone belt. The results are consistent with findings along the granitoid-greenstone contacts some 40 km west of the Steynsdorp area, and they indicate the regional extent of this metamorphic core complex and the allochthonous nature of the high-pressure, low-temperature terrane with respect to the rest of the greenstone belt. This also implies that the 3.45 Ga suite of TTG rocks south of the Barberton greenstone belt is unlikely to have represented the source of similar-aged volcanic rocks in the belt. More voluminous and larger extent of 3.45 Ga TTG plutonism represents a significant early crust-forming event in the Barberton terrain, the evidence of which has been obliterated during later episodes of tectonomagmatic recycling. The existence of the high-pressure, low-temperature terrane in the southern Barberton granitoid-greenstone terrain indicates the presence of a cool and rigid continental nucleus in the Mesoarchean around which plate-tectonic processes could initiate.

Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa

Although plate tectonics is the central geological process of the modern Earth, its form and existence during the Archaean era (4.0-2.5 Gyr ago) are disputed. The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 degrees C km(-1) or less, have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients are greater than 25 degrees C km(-1)). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites and circumstantial evidence for subduction processes, including possible accretion-related structures, has been reported in Archaean terrains. The lack of spatially and temporally well-constrained high-pressure, low-temperature metamorphism continues, however, to cast doubt on the relevance of subduction-driven tectonics during the first 1.5 Gyr of the Earth's history. Here we report garnet-albite-bearing mineral assemblages that record pressures of 1.2-1.5 GPa at temperatures of 600-650 degrees C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12-15 degrees C-similar to those found in recent subduction zones-that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt. These high-pressure, low-temperature conditions represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.

Progressive adjustments of ascent and emplacement controls during incremental construction of the 3.1 Ga Heerenveen batholith, South Africa

The Heerenveen batholith is part of a suite of areally extensive, shallow-crustal granitoid plutons intruded during the last regional phase of tectonism and NW-SE subhorizontal shortening recorded in the Mesoarchean Barberton granitoid-greenstone terrain of South Africa at 3.1 Ga. Intrusive relationships allow at least four main successively emplaced intrusive stages to be distinguished. Each of these shows distinct geometries and intrusive styles that provide evidence for the progressive change of emplacement controls during the incremental construction of the Heerenveen batholith. The earliest sheet-like granitoids intruded as foliation-parallel sills along the shallowly dipping basement gneissosity, emphasizing the role of favourably inclined pre-existing wall-rock anisotropies for granite emplacement during the early stages of pluton assembly. Continued sheeting and coalescence of sheets provided the thermal ground preparation that led to the formation of larger, coherent magma bodies and the main phase of homogeneous, commonly megacrystic granites. These megacrystic granites form the central parts of the Heerenveen batholith, and are interpreted to represent steady-state magma chambers. The introduction of the rheologically weaker melt bodies into the shallow crust resulted in the nucleation of conjugate synmagmatic transpressive shear zones around the central granites. The shear zones correspond to several km-wide zones of shear zone-parallel granite sheeting. This stage marks a dramatic switch in emplacement styles. While the initial stages of magma emplacement were largely determined by factors external to the magma, most importantly the pre-existing wall-rock anisotropies, subsequent stages are dominated by factors intrinsic to the magma, namely strain localization and partitioning along melt-bearing zones during syntectonic plutonism. The associated melt transfer along these zones is independent of pre-existing structures and mainly related to buoyancy- and strain-induced melt ascent. The last granites of the Heerenveen batholith are post-tectonic. They intrude as either plugs or stocks of seemingly random orientation, but display a clear control by wall-rock anisotropies where they are in contact with the country rocks. On a regional scale, the different phases of the Heerenveen batholith describe an overall zonation of central homogeneous granites enveloped by composite, sheeted and sheared margins. This pattern is typical for most of the large 3.1 Ga granite batholiths in the Barberton granitoid-greenstone terrain. We suggest that the sequence of progressively changing emplacement controls and the formation of steady-state magma chambers described here for the Heerenveen batholith may be of wider application to other zoned and/or incrementally assembled batholiths.

Metamorphism and exhumation of the basal parts of the Barberton greenstone belt, South Africa: Constraining the rates of Mesoarchaean tectonism

The Paleo- to Mesoarchaean Barberton granitoid-greenstone terrain of South Africa consists of two main components: the low-grade metamorphic supracrustal greenstone sequence of the Barberton greenstone belt in the north and a high-grade metamorphic granitoid-gneiss terrain to the south. The boundary between the two different domains corresponds to the highly tectonized, amphibolite-facies rocks of the Theespruit Formation that occur along the margins of the greenstone belt. These rocks record high-P, low-T peak metamorphic conditions of 7.4±1.0kbar and 560±20°C that are very similar to estimates from other areas of the high-grade terrain and were attained during the main phase of terrain accretion in the greenstone belt at 3230Ma. In contrast, the greenstone sequence ca. 4km to the north only records low greenschist-facies metamorphism, indicating that a metamorphic break of ca. 18km exists between the high-grade terrain and the greenstone belt. The main phase of deformation in the Theespruit Formation was initiated under peak metamorphic conditions and continued during retrogression. Retrograde P–T estimates and mineral reactions indicate that retrogression involved near-isothermal decompression of ca. 4kbar prior to cooling into the greenschist-facies, suggesting that the fabric in these rocks is an exhumation fabric that accommodated the juxtaposition of the high-grade terrain against the greenstone belt. Geochronological constraints, combined with the depths of burial indicate that exhumation of the high-grade rocks occurred at rates of 2–5mm/a and are comparable to the exhumation rates of crustal rocks in younger orogenic environments. The extremely low apparent geothermal gradients of ca. 20°C/km that are recorded in the high-grade terrain are inconsistent with models of a hotter and weaker crustal environment in the Archaean. Rather, the depths of burial and structural integrity of this terrain suggest that the Mesoarchaean crust was cold and rigid enough to allow tectonic stacking and crustal overthickening and had a rheology similar to modern continental crust.

The oldest part of the Barberton granitoid-greenstone terrain, South Africa: evidence for crust formation between 3.5 and 3.7 Ga

Many stratigraphic and age relationships in the southern part of the Barberton greenstone belt (BGB) remain unresolved due to strong deformation including thrusting, nappe stacking and strike-slip faulting. A relatively undisturbed sequence from the lower Onverwacht Group (∼ 3.48-3.45 Ga), followed by the upper Onverwacht (∼ 3.42-3.3 Ga), the Fig Tree Group (∼ 3.26-3.23 Ga) and the Moodies Group (> 3.22 Ga) was established by single zircon dating using various techniques, but the position of the Theespruit Formation is still uncertain. Using the single zircon evaporation and the vapour digestion techniques we obtained remarkably uniform 207Pb/ 206Pb and UPb ages of 3544 ± 3 to 3547 ± 3 Ma for felsic rocks mapped as Theespruit in the Steynsdorp Anticline of the southeastern BGB, some 100 Ma older than all other dated greenstone units. These rocks were intruded by the 3502–3511 Ma old Steynsdorp TTG pluton containing zircon xenocrysts as old as 3553 ± 4 Ma. A 3.5 Ga granodiorite plug intrusive into metavolcanics of the Komati Formation (lower Onverwacht Group) contained two 3702 ± 2 Ma zircon xenocrysts, the oldest so far measured in the Barberton-Swaziland area and testifying to the presence of very ancient crust in the region. The Theespruit felsic metavolcanics have ϵ Nd(t) values between + 1.1 and − 1.1 and Nd T DM model ages between 3.5 and 3.7 Ga, suggesting variable contamination of their protoliths with older continental crust. The above zircon ages extend the history of the BGB back to ∼ 3.55 Ga. We suggest that the area of the Steynsdorp Anticline constitutes the oldest nucleus of the BGB onto which successively younger units were tectonically and magmatically accreted. We also speculate that the mafic-felsic volcanic units of the southern BGB may perhaps represent distinct oceanic plateaux, rather than ocean floor material, which amalgamated between 3.55 and 3.42 Ga ago. Our data support the concept that the BGB consists of a number of discrete, fault-bounded terranes, and that large-scale lithological correlations are therefore not justified.

Least radiogenic terrestrial leads: A comment

The Heerenveen batholith is part of a suite of areally extensive, shallow-crustal granitoid plutons intruded during the last regional phase of tectonism and NW-SE subhorizontal shortening recorded in the Mesoarchean Barberton granitoid-greenstone terrain of South Africa at 3.1 Ga. Intrusive relationships allow at least four main successively emplaced intrusive stages to be distinguished. Each of these shows distinct geometries and intrusive styles that provide evidence for the progressive change of emplacement controls during the incremental construction of the Heerenveen batholith. The earliest sheet-like granitoids intruded as foliation-parallel sills along the shallowly dipping basement gneissosity, emphasizing the role of favourably inclined pre-existing wall-rock anisotropies for granite emplacement during the early stages of pluton assembly. Continued sheeting and coalescence of sheets provided the thermal ground preparation that led to the formation of larger, coherent magma bodies and the main phase of homogeneous, commonly megacrystic granites. These megacrystic granites form the central parts of the Heerenveen batholith, and are interpreted to represent steady-state magma chambers. The introduction of the rheologically weaker melt bodies into the shallow crust resulted in the nucleation of conjugate synmagmatic transpressive shear zones around the central granites. The shear zones correspond to several km-wide zones of shear zone-parallel granite sheeting. This stage marks a dramatic switch in emplacement styles. While the initial stages of magma emplacement were largely determined by factors external to the magma, most importantly the pre-existing wall-rock anisotropies, subsequent stages are dominated by factors intrinsic to the magma, namely strain localization and partitioning along melt-bearing zones during syntectonic plutonism. The associated melt transfer along these zones is independent of pre-existing structures and mainly related to buoyancy- and strain-induced melt ascent. The last granites of the Heerenveen batholith are post-tectonic. They intrude as either plugs or stocks of seemingly random orientation, but display a clear control by wall-rock anisotropies where they are in contact with the country rocks.On a regional scale, the different phases of the Heerenveen batholith describe an overall zonation of central homogeneous granites enveloped by composite, sheeted and sheared margins. This pattern is typical for most of the large 3.1 Ga granite batholiths in the Barberton granitoid-greenstone terrain. We suggest that the sequence of progressively changing emplacement controls and the formation of steady-state magma chambers described here for the Heerenveen batholith may be of wider application to other zoned and/or incrementally assembled batholiths.